I heard Messenger of Allah ﷺ saying regarding Ramadan, "Whoever prayed at night in it (the month of Ramadan) out of sincere Faith and hoping for a reward from Allah, then all his previous sins will be forgiven."  

سَمِعْتُ رَسُولَ اللَّهِ ﷺ يَقُولُ لِرَمَضَانَ مَنْ قَامَهُ إِيمَانًا وَاحْتِسَابًا غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ  

Messenger of Allah ﷺ said, "Whoever prayed at night the whole month of Ramadan out of sincere Faith and hoping for a reward from Allah, then all his previous sins will be forgiven." Ibn Shihab (a sub-narrator) said, "Messenger of Allah ﷺ died and the people continued observing that (i.e. Nawafil offered individually, not in congregation), and it remained as it was during the Caliphate of Abu Bakr and in the early days of 'Umar's Caliphate."  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ مَنْ قَامَ رَمَضَانَ إِيمَانًا وَاحْتِسَابًا غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ قَالَ ابْنُ شِهَابٍ فَتُوُفِّيَ رَسُولُ اللَّهِ ﷺ وَالأَمْرُ عَلَى ذَلِكَ ثُمَّ كَانَ الأَمْرُ عَلَى ذَلِكَ فِي خِلاَفَةِ أَبِي بَكْرٍ وَصَدْرًا مِنْ خِلاَفَةِ عُمَرَ ؓ  

I went out in the company of ʿUmar b. al-Khaṭṭāb one night in Ramaḍān to the mosque and found the people praying in different groups. A man praying alone or a man praying with a little group behind him. So, ʿUmar said: In my opinion, it would better to collect them under the leadership of one reciter. So, he made up his mind to congregate them behind Ubay b. Kaʿb. Then on another night, I went again in his company and the people were praying behind their reciter. On that, ʿUmar remarked: What an excellent *bidʿah* (innovation) this is; but the prayer which they do not perform and sleep at its time is better than the one they are offering. ˹What˺ he meant is the prayer in the last part of the night, (i.e. Tahajjud) ˹In those days˺ people used to pray in the early part of the night.  

خَرَجْتُ مَعَ عُمَرَ بْنِ الْخَطَّابِ ؓ لَيْلَةً فِي رَمَضَانَ إِلَى الْمَسْجِدِ فَإِذَا النَّاسُ أَوْزَاعٌ مُتَفَرِّقُونَ، يُصَلِّي الرَّجُلُ لِنَفْسِهِ وَيُصَلِّي الرَّجُلُ فَيُصَلِّي بِصَلاَتِهِ الرَّهْطُ۔ فَقَالَ عُمَرُ: إِنِّي أَرَى لَوْ جَمَعْتُ هَؤُلاَءِ عَلَى قَارِئٍ وَاحِدٍ لَكَانَ أَمْثَلَ۔ ثُمَّ عَزَمَ فَجَمَعَهُمْ عَلَى أُبَىِّ بْنِ كَعْبٍ، ثُمَّ خَرَجْتُ مَعَهُ لَيْلَةً أُخْرَى وَالنَّاسُ يُصَلُّونَ بِصَلاَةِ قَارِئِهِمْ۔ قَالَ عُمَرُ: نِعْمَ الْبِدْعَةُ هَذِهِ! وَالَّتِي يَنَامُونَ عَنْهَا أَفْضَلُ مِنَ الَّتِي يَقُومُونَ، يُرِيدُ آخِرَ اللَّيْلِ، وَكَانَ النَّاسُ يَقُومُونَ أَوَّلَهُ۔  

(the wife of the Prophet) Messenger of Allah ﷺ used to pray (at night) in Ramadan.  

عَنْ عَائِشَةَ ؓ زَوْجِ النَّبِيِّ ﷺ أَنَّ رَسُولَ اللَّهِ ﷺ صَلَّى وَذَلِكَ فِي رَمَضَانَ  

That he was informed by ʿAisha, "Messenger of Allah ﷺ went out in the middle of the night and prayed in the mosque and some men prayed behind him. In the morning, the people spoke about it and then a large number of them gathered and prayed behind him (on the second night). In the next morning the people again talked about it and on the third night the mosque was full with a large number of people. Messenger of Allah ﷺ came out and the people prayed behind him. On the fourth night the Mosque was overwhelmed with people and could not accommodate them, but the Prophet ﷺ came out (only) for the morning prayer. When the morning prayer was finished he recited Tashah-hud and (addressing the people) said, "Amma ba'du, your presence was not hidden from me but I was afraid lest the night prayer (Qiyam) should be enjoined on you and you might not be able to carry it on." So, Allah's Apostle died and the situation remained like that (i.e. people prayed individually). "  

أَنَّ رَسُولَ اللَّهِ ﷺ خَرَجَ لَيْلَةً مِنْ جَوْفِ اللَّيْلِ فَصَلَّى فِي الْمَسْجِدِ وَصَلَّى رِجَالٌ بِصَلاَتِهِ فَأَصْبَحَ النَّاسُ فَتَحَدَّثُوا فَاجْتَمَعَ أَكْثَرُ مِنْهُمْ فَصَلَّوْا مَعَهُ فَأَصْبَحَ النَّاسُ فَتَحَدَّثُوا فَكَثُرَ أَهْلُ الْمَسْجِدِ مِنَ اللَّيْلَةِ الثَّالِثَةِ فَخَرَجَ رَسُولُ اللَّهِ ﷺ فَصَلَّى فَصَلَّوْا بِصَلاَتِهِ فَلَمَّا كَانَتِ اللَّيْلَةُ الرَّابِعَةُ عَجَزَ الْمَسْجِدُ عَنْ أَهْلِهِ حَتَّى خَرَجَ لِصَلاَةِ الصُّبْحِ فَلَمَّا قَضَى الْفَجْرَ أَقْبَلَ عَلَى النَّاسِ فَتَشَهَّدَ ثُمَّ قَالَ أَمَّا بَعْدُ فَإِنَّهُ لَمْ يَخْفَ عَلَىَّ مَكَانُكُمْ وَلَكِنِّي خَشِيتُ أَنْ تُفْتَرَضَ عَلَيْكُمْ فَتَعْجِزُوا عَنْهَا فَتُوُفِّيَ رَسُولُ اللَّهِ ﷺ وَالأَمْرُ عَلَى ذَلِكَ  

that he asked ʿAisha "How was the prayer of Messenger of Allah ﷺ in Ramadan?" She replied, "He did not pray more than eleven rakʿat in Ramadan or in any other month. He used to pray four rakʿat ---- let alone their beauty and length----and then he would pray four ----let alone their beauty and length ---- and then he would pray three rakʿat (witr)." She added, "I asked, 'O Messenger of Allah ﷺ! Do you sleep before praying the witr?' He replied, 'O ʿAisha! My eyes sleep but my heart does not sleep."  

أَنَّهُ سَأَلَ عَائِشَةَ ؓ كَيْفَ كَانَتْ صَلاَةُ رَسُولِ اللَّهِ ﷺ فِي رَمَضَانَ فَقَالَتْ مَا كَانَ يَزِيدُ فِي رَمَضَانَ وَلاَ فِي غَيْرِهَا عَلَى إِحْدَى عَشْرَةَ رَكْعَةً يُصَلِّي أَرْبَعًا فَلاَ تَسَلْ عَنْ حُسْنِهِنَّ وَطُولِهِنَّ ثُمَّ يُصَلِّي أَرْبَعًا فَلاَ تَسَلْ عَنْ حُسْنِهِنَّ وَطُولِهِنَّ ثُمَّ يُصَلِّي ثَلاَثًا فَقُلْتُ يَا رَسُولَ اللَّهِ أَتَنَامُ قَبْلَ أَنْ تُوتِرَ قَالَ يَا عَائِشَةُ إِنَّ عَيْنَىَّ تَنَامَانِ وَلاَ يَنَامُ قَلْبِي