I never saw the Prophet ﷺ seeking to fast on a day that he favored more than another except this day, the day of 'Ashura', and this month, meaning the month of Ramadan.
مَا رَأَيْتُ النَّبِيَّ ﷺ يَتَحَرَّى صِيَامَ يَوْمٍ فَضَّلَهُ عَلَى غَيْرِهِ إِلاَّ هَذَا الْيَوْمَ يَوْمَ عَاشُورَاءَ وَهَذَا الشَّهْرَ يَعْنِي شَهْرَ رَمَضَانَ
The Prophet ﷺ said, "Time has taken its original shape which it had when Allah created the Heavens and the Earth. The year is of twelve months, four of which are sacred, and out of these (four) three are in succession, i.e. Dhul-Qa'da, Dhul-Hijja and Al-Muharram, and the fourth is Rajab which is named after the Mudar tribe, between (the month of) Jumaida (ath-thania) and Sha'ban." Then the Prophet ﷺ asked, "Which is this month?" We said, "Allah and His Apostle know better." On that the Prophet ﷺ kept quiet so long that we thought that he might name it with another name. Then the Prophet ﷺ said, "Isn't it the month of Dhul-Hijja?" We replied, "Yes." Then he said, "Which town is this?" "We replied, "Allah and His Apostle know better." On that he kept quiet so long that we thought that he might name it with another name. Then he said, "Isn't it the town of Mecca?" We replied, "Yes, " Then he said, "Which day is today?" We replied, "Allah and His Apostle know better." He kept quiet so long that we thought that he might name it with another name. Then he said, "Isn't it the day of An- Nahr (i.e. sacrifice)?" We replied, "Yes." He said, "So your blood, your properties, (The sub-narrator Muhammad said, 'I think the Prophet ﷺ also said: And your honor..) are sacred to one another like the sanctity of this day of yours, in this city of yours, in this month of yours; and surely, you will meet your Lord, and He will ask you about your deeds. Beware! Do not become infidels after me, cutting the throats of one another. It is incumbent on those who are present to convey this message (of mine) to those who are absent. May be that some of those to whom it will be conveyed will understand it better than those who have actually heard it." (The sub-narrator, Muhammad, on remembering that narration, used to say, "Muhammad spoke the truth!") He (i.e. Prophet) then added twice, "No doubt! Haven't I conveyed (Allah's Message) to you?"
عَنِ النَّبِيِّ ﷺ قَالَ الزَّمَانُ قَدِ اسْتَدَارَ كَهَيْئَةِ يَوْمَ خَلَقَ السَّمَوَاتِ وَالأَرْضَ السَّنَةُ اثْنَا عَشَرَ شَهْرًا مِنْهَا أَرْبَعَةٌ حُرُمٌ ثَلاَثَةٌ مُتَوَالِيَاتٌ ذُو الْقَعْدَةِ وَذُو الْحِجَّةِ وَالْمُحَرَّمُ وَرَجَبُ مُضَرَ الَّذِي بَيْنَ جُمَادَى وَشَعْبَانَ أَىُّ شَهْرٍ هَذَا قُلْنَا اللَّهُ وَرَسُولُهُ أَعْلَمُ فَسَكَتَ حَتَّى ظَنَنَّا أَنَّهُ سَيُسَمِّيهِ بِغَيْرِ اسْمِهِ قَالَ أَلَيْسَ ذُو الْحِجَّةِ قُلْنَا بَلَى قَالَ فَأَىُّ بَلَدٍ هَذَا قُلْنَا اللَّهُ وَرَسُولُهُ أَعْلَمُ فَسَكَتَ حَتَّى ظَنَنَّا أَنَّهُ سَيُسَمِّيهِ بِغَيْرِ اسْمِهِ قَالَ أَلَيْسَ الْبَلْدَةَ قُلْنَا بَلَى قَالَ فَأَىُّ يَوْمٍ هَذَا قُلْنَا اللَّهُ وَرَسُولُهُ أَعْلَمُ فَسَكَتَ حَتَّى ظَنَنَّا أَنَّهُ سَيُسَمِّيهِ بِغَيْرِ اسْمِهِ قَالَ أَلَيْسَ يَوْمَ النَّحْرِ قُلْنَا بَلَى قَالَ فَإِنَّ دِمَاءَكُمْ وَأَمْوَالَكُمْ قَالَ مُحَمَّدٌ وَأَحْسِبُهُ قَالَ وَأَعْرَاضَكُمْ عَلَيْكُمْ حَرَامٌ كَحُرْمَةِ يَوْمِكُمْ هَذَا فِي بَلَدِكُمْ هَذَا فِي شَهْرِكُمْ هَذَا وَسَتَلْقَوْنَ رَبَّكُمْ فَسَيَسْأَلُكُمْ عَنْ أَعْمَالِكُمْ أَلاَ فَلاَ تَرْجِعُوا بَعْدِي ضُلاَّلاً يَضْرِبُ بَعْضُكُمْ رِقَابَ بَعْضٍ أَلاَ لِيُبَلِّغِ الشَّاهِدُ الْغَائِبَ فَلَعَلَّ بَعْضَ مَنْ يُبَلَّغُهُ أَنْ يَكُونَ أَوْعَى لَهُ مِنْ بَعْضِ مَنْ سَمِعَهُ فَكَانَ مُحَمَّدٌ إِذَا ذَكَرَهُ يَقُولُ صَدَقَ مُحَمَّدٌ ﷺ ثُمَّ قَالَ أَلاَ هَلْ بَلَّغْتُ مَرَّتَيْنِ
he attributed this to the Prophet ﷺ: He was asked which prayer is best after the obligatory prayers and which fasting is best after the month of Ramadan? He said: (1) The best prayer after the obligatory prayers is the one offered in the middle of the night, and (2) the best fast after the month of Ramadan is the fast in the Month of Allah: al-Muharram.
سُئِلَ أَىُّ الصَّلاَةِ أَفْضَلُ بَعْدَ الْمَكْتُوبَةِ وَأَىُّ الصِّيَامِ أَفْضَلُ بَعْدَ شَهْرِ رَمَضَانَ فَقَالَ أَفْضَلُ الصَّلاَةِ بَعْدَ الصَّلاَةِ الْمَكْتُوبَةِ الصَّلاَةُ فِي جَوْفِ اللَّيْلِ وَأَفْضَلُ الصِّيَامِ بَعْدَ شَهْرِ رَمَضَانَ صِيَامُ شَهْرِ اللَّهِ الْمُحَرَّمِ
When the Prophet ﷺ on the day of 'Ashurah and commanded us to fast on it, they (i.e. Companions) said: Messenger of Allah, this is a day which is considered great by Jews and Christians ? The Messenger of Allah ﷺ said: When the next year comes, we shall fast on the 9th of Muharram. But the next year the Messenger of Allah ﷺ breathed his last.
سَمِعْتُ عَبْدَ اللَّهِ بْنَ عَبَّاسٍ يَقُولُ حِينَ صَامَ النَّبِيُّ ﷺ يَوْمَ عَاشُورَاءَ وَأَمَرَنَا بِصِيَامِهِ قَالُوا يَا رَسُولَ اللَّهِ إِنَّهُ يَوْمٌ تُعَظِّمُهُ الْيَهُودُ وَالنَّصَارَى فَقَالَ رَسُولُ اللَّهِ ﷺ فَإِذَا كَانَ الْعَامُ الْمُقْبِلُ صُمْنَا يَوْمَ التَّاسِعِ فَلَمْ يَأْتِ الْعَامُ الْمُقْبِلُ حَتَّى تُوُفِّيَ رَسُولُ اللَّهِ ﷺ
the Messenger of Allah said: "The most virtuous fasting after the month of Ramadan is Allah's month Al-Muharram."
قَالَ رَسُولُ اللَّهِ ﷺ أَفْضَلُ الصِّيَامِ بَعْدَ صِيَامِ شَهْرِ رَمَضَانَ شَهْرُ اللَّهِ الْمُحَرَّمُ
[Machine] The Prophet ﷺ crossed paths with some people from the Jews who were fasting on the day of Ashura. He asked, "What is the significance of this fasting?" They replied, "This is the day on which Allah saved Moses and the Children of Israel from drowning, and drowned Pharaoh. On this day, the ark also came to rest on Mount Judi, so Noah and Moses fasted in gratitude to Allah." Upon hearing this, the Prophet ﷺ said, "I have a greater right to Moses than they do, and this fasting is more deserving for me." He then commanded his companions to observe the fast.
مَرَّ النَّبِيُّ ﷺ بِأُنَاسٍ مِنَ الْيَهُودِ قَدْ صَامُوا يَوْمَ عَاشُورَاءَ فَقَالَ مَا هَذَا مِنَ الصَّوْمِ؟ قَالُوا هَذَا الْيَوْمُ الَّذِي نَجَّى اللهُ مُوسَى وَبَنِي إِسْرَائِيلَ مِنَ الْغَرَقِ وَغَرَّقَ فِيهِ فِرْعَوْنَ وَهَذَا يَوْمُ اسْتَوَتْ فِيهِ السَّفِينَةُ عَلَى الْجُودِيِّ فَصَامَ نُوحٌ وَمُوسَى شُكْرًا لِلَّهِ فَقَالَ النَّبِيُّ ﷺ أَنَا أَحَقُّ بِمُوسَى وَأَحَقُّ بِصَوْمِ هَذَا الْيَوْمِ فَأَمَرَ أَصْحَابَهُ بِالصَّوْمِ
[Machine] "That the Messenger of Allah ﷺ said: 'If I live until the next year, I will certainly fast the ninth day (of Muharram),' meaning the day of Ashura."
أَنَّ رَسُولَ اللهِ ﷺ قَالَ لَئِنْ عِشْتُ قَالَ رَوْحٌ لَئِنْ سَلِمْتُ إِلَى قَابِلٍ لَأَصُومَنَّ الْيَوْمَ التَّاسِعَ يَعْنِي عَاشُورَاءَ
[Machine] The Messenger of Allah ﷺ said: "Fast on the day of Ashura and be different from the Jews. Fast a day before it or a day after it."
قَالَ رَسُولُ اللهِ ﷺ صُومُوا يَوْمَ عَاشُورَاءَ وَخَالِفُوا فِيهِ الْيَهُودَ صُومُوا قَبْلَهُ يَوْمًا أَوْ بَعْدَهُ يَوْمًا
[Machine] The Messenger of Allah ﷺ said: "Rajab is a great month in which Allah multiplies the rewards. So whoever fasts for a day in Rajab, it is as if he has fasted for a whole year. And whoever fasts for seven days, the seven gates of Hell will be closed to him. And whoever fasts for eight days, the eight gates of Paradise will be opened for him. And whoever fasts for ten days, Allah will grant him whatever he asks for. And whoever fasts for fifteen days, a caller will announce in the heavens: 'May Allah forgive what has passed for you, so start anew.' And whoever does more, Allah will increase it. And in Rajab, Allah carried Noah in the ark, so he fasted in Rajab and commanded his companions to fast. The ark set sail with them for six months, until they landed on Mount Judi. And Noah and those with him expressed gratitude to Allah. And on the day of Ashura, Allah split the sea for the children of Israel. And on the day of Ashura, Allah accepted the repentance of Adam and the city of Yunus. And in Rajab, Ibrahim ﷺ was born."
قَالَ رَسُولُ اللهِ ﷺ «رَجَبٌ شَهْرٌ عَظِيمٌ يُضَاعِفُ اللهُ فِيهِ الْحَسَنَاتِ فَمَنْ صَامَ يَوْمًا مِنْ رَجَبٍ فَكَأَنَّمَا صَامَ سَنَةً وَمَنْ صَامَ مِنْهُ سَبْعَةَ أَيَّامٍ غُلِّقَتْ عَنْهُ سَبْعَةُ أَبْوَابِ جَهَنَّمَ وَمَنْ صَامَ مِنْهُ ثَمَانِيَةَ أَيَّامٍ فُتِحَتْ لَهُ ثَمَانِيَةُ أَبْوَابِ الْجَنَّةِ وَمَنْ صَامَ مِنْهُ عَشَرَةَ أَيَّامٍ لَمْ يَسْأَلِ اللهَ شَيْئًا إِلَّا أَعْطَاهُ إِيَّاهُ وَمَنْ صَامَ مِنْهُ خَمْسَةَ عَشَرَ يَوْمًا نَادَى مُنَادٍ فِي السَّمَاءِ قَدْ غُفِرَ لَكَ مَا مَضَى فَاسْتَئْنِفِ الْعَمَلَ وَمَنْ زَادَ زَادَهُ اللهُ ﷻ وَفِي رَجَبٍ حَمَلَ اللهُ نُوحًا فِي السَّفِينَةِ فَصَامَ رَجَبًا وَأَمَرَ مَنْ مَعَهُ أَنْ يَصُومُوا فَجَرَتْ بِهِمُ السَّفِينَةُ سِتَّةَ أَشْهُرٍ آخِرُ ذَلِكَ يَوْمُ عَاشُورَاءَ أُهْبِطَ عَلَى الْجُودِيِّ فَصَامَ نُوحٌ وَمَنْ مَعَهُ وَالْوَحْشُ شُكْرًا لِلَّهِ ﷻ وَفِي يَوْمِ عَاشُورَاءَ أفْلَقَ اللهُ الْبَحْرَ لِبَنِي إِسْرَائِيلَ وَفِي يَوْمِ عَاشُورَاءَ تَابَ اللهُ ﷻ عَلَى آدَمَ ﷺ وَعَلَى مَدِينَةِ يُونُسَ وَفِيهِ وُلِدَ إِبْرَاهِيمُ ﷺ»
"مَنْ صَامَ يَوْمًا مِنْ رَجَبٍ كانَ كَصِيَامِ سَنَة، وَمَن صَامَ سَبْعَةَ أَيَّامٍ غُلِّقَتْ عَنْهُ سَبْعَةُ أَبْوَاب جَهَنَّمَ، وَمَن صَامَ ثَمَانِيَةَ أَيَّامٍ فُتِّحَتْ لَهُ ثَمَانِيَةُ أَبْوَابِ الجَنَّةِ، وَمَنْ صَامَ عَشْرَةَ أَيَّامٍ لَمْ يَسْألِ اللهَ شَيئًا إِلا أَعْطَاهُ، وَمَن صَامَ خَمْسَةَ عَشَرَ يَوْمًا نَادَى مُنَادٍ مِنَ السَّمَاءِ: قَدْ غَفَرْتُ لَكَ مَا سَلَفَ، فَاسْتَأنِف العَمَلَ، قَدْ بَدَّلتُ سيئاتِكَ حَسَنَات، وَمَن زَادَ زَادَهُ اللهُ، وفِي رَجَبٍ حُمِلَ نُوح بِالسَّفِينة وَصَامَ نُوحٌ وَأَمَرَ مَنْ مَعَهُ أَنْ يَصُومُوا، وَجَرَت بِهِم السفِينَةُ سِتَّةَ أَشْهُر إِلَى آخِرِ ذَلِكَ لِعَشْر خَلَوْنَ مِن المُحَرمِ".
"رجبٌ شَهْرٌ عظيمٌ، يضاعِفُ اللَّه فيهِ الحسنات، لمنْ صام يوْمًا مِنْ رجب فَكَأنَّما صام سنةً، ومنْ صام مِنْه سبْعةَ أَيَّام غُلِّقَتْ عَنْه أبْوابُ جهنَّمَ، ومنْ صام منْهُ ثَمانِيةَ أَيَّامٍ فُتِّحتْ لَه ثَمانِيةُ أَبْواب الجنِّةِ، ومنْ صام مِنهُ عشرةَ أَيَّامٍ لَمْ يسْأَل اللَّهَ شَيْئًا إِلَّا أَعْطَاه، ومنْ صام مِنْه خَمْسة عشَر يوْمًا نَادى منَاد مِنَ السَّماءِ، قَدْ غُفِر لَكَ ما مضَى فَاستَأْنْفِ العملَ، ومنْ زاد زَاده اللَّه، وفى رجبٍ حَملَ اللَّه نُوحًا في السَّفينةَ فَصام رجب، وأَمر مَنْ معه أَنْ يصوموا فَجرتْ بهمْ السَّفِينَةُ سِتَّةَ أَشْهر، آخِرُ ذَلَكَ الْيَوْم يوْم عاشُوراءَ، اهْبطَ علَى الجودِىِّ، فَصَام نُوحٌ ومنْ معه، والْوحْشُ شُكْرًا للَّهِ ﷻ وفى يَوْم عَاشُورَاءَ فلَقَ اللَّهُ البَحْرَ لبنى إِسْرائِيل وفى يوم عاشُوراءَ تَاب اللَّه علَى آدم وعلَى مدِينةِ يونُس، وفِيهِ وُلِد إِبْراهِيم".