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tirmidhi:549 – Hannād b. al-Sarī > Abū Muʿāwiyah > ʿĀṣim al-Aḥwal > ʿIkrimah > Ibn ʿAbbās

"The Messenger of Allah traveled on a journey and he prayed two Rak'ah for nineteen days." Ibn Abbas said: "So when we would stay somewhere for nineteen (days) we would pray two Rak'ah , and if we stayed longer than that we would complete the Salat."  

Abū ʿĪsá said that this Ḥadīth is Ḥasan-Ṣaḥīḥ.
الترمذي:٥٤٩ – حَدَّثَنَا هَنَّادُ بْنُ السَّرِيِّ حَدَّثَنَا أَبُو مُعَاوِيَةَ عَنْ عَاصِمٍ الأَحْوَلِ عَنْ عِكْرِمَةَ عَنِ ابْنِ عَبَّاسٍ قَالَ

سَافَرَ رَسُولُ اللَّهِ ﷺ سَفَرًا فَصَلَّى تِسْعَةَ عَشَرَ يَوْمًا رَكْعَتَيْنِ رَكْعَتَيْنٍ قَالَ ابْنُ عَبَّاسٍ فَنَحْنُ نُصَلِّي فِيمَا بَيْنَنَا وَبَيْنَ تِسْعَ عَشْرَةَ رَكْعَتَيْنِ رَكْعَتَيْنِ فَإِذَا أَقَمْنَا أَكْثَرَ مِنْ ذَلِكَ صَلَّيْنَا أَرْبَعًا  

قَالَ أَبُو عِيسَى هَذَا حَدِيثٌ حَسَنٌ صَحِيحٌ

See similar narrations below:

Collected by Ibn Mājah, Aḥmad, Bayhaqī
ibnmajah:1075 – Muḥammad b. ʿAbd al-Malik b. Abū al-Shawārib > ʿAbd al-Wāḥid b. Ziyād > ʿĀṣim al-Aḥwal > ʿIkrimah > Ibn ʿAbbās

“The Messenger of Allah ﷺ stayed for nineteen days in which he shortened his prayer to two Rak’ah. So, whenever we stayed for nineteen days we would shorten our prayer to two Rak’ah, but if we stayed more than that we would pray four Rak’ah.”  

ابن ماجة:١٠٧٥ – حَدَّثَنَا مُحَمَّدُ بْنُ عَبْدِ الْمَلِكِ بْنِ أَبِي الشَّوَارِبِ حَدَّثَنَا عَبْدُ الْوَاحِدِ بْنُ زِيَادٍ حَدَّثَنَا عَاصِمٌ الأَحْوَلُ عَنْ عِكْرِمَةَ عَنِ ابْنِ عَبَّاسٍ قَالَ

أَقَامَ رَسُولُ اللَّهِ ﷺ تِسْعَةَ عَشَرَ يَوْمًا يُصَلِّي رَكْعَتَيْنِ رَكْعَتَيْنِ فَنَحْنُ إِذَا أَقَمْنَا تِسْعَةَ عَشَرَ يَوْمًا نُصَلِّي رَكْعَتَيْنِ رَكْعَتَيْنِ فَإِذَا أَقَمْنَا أَكْثَرَ مِنْ ذَلِكَ صَلَّيْنَا أَرْبَعًا  

ahmad:1958 – Abū Muʿāwiyah > ʿĀṣim al-Aḥwal > ʿIkrimah > Ibn ʿAbbās

“The Messenger of Allah ﷺ stayed for nineteen days in which he shortened his prayer to two Rak’ah. So, whenever we stayed for nineteen days we would shorten our prayer to two Rak’ah, but if we stayed more than that we would pray four Rak’ah.” (Using translation from Ibn Mājah 1075)   

أحمد:١٩٥٨ – حَدَّثَنَا أَبُو مُعَاوِيَةَ حَدَّثَنَا عَاصِمٌ الْأَحْوَلُ عَنْ عِكْرِمَةَ عَنِ ابْنِ عَبَّاسٍ قَالَ

سَافَرَ رَسُولُ اللهِ ﷺ سَفَرًا فَأَقَامَ تِسْعَ عَشْرَةَ يُصَلِّي رَكْعَتَيْنِ رَكْعَتَيْنِ قَالَ ابْنُ عَبَّاسٍ فَنَحْنُ إِذَا سَافَرْنَا فَأَقَمْنَا تِسْعَ عَشْرَةَ صَلَّيْنَا رَكْعَتَيْنِ رَكْعَتَيْنِ فَإِذَا أَقَمْنَا أَكْثَرَ مِنْ ذَلِكَ صَلَّيْنَا أَرْبَعًا  

bayhaqi:5463 – [Chain 1] Muḥammad b. ʿAbdullāh al-Bisṭāmī > Abū Bakr al-Ismāʿīlī > Abū Yaʿlá > Abū Khaythamah And Surayj b. Yūnus > Abū Muʿāwiyah [Chain 2] Muḥammad > Abū Bakr > al-Jawzī > Mujāhid b. Mūsá And Yaʿqūb al-Dawraqī Wa-al-Faḍl b. al-Ṣabbāḥ Wayūsuf > Abū Muʿāwiyah Wahadhā Ḥadīth al-Jawzī > ʿĀṣim al-Aḥwal > ʿIkrimah > Ibn ʿAbbās

[Machine] The Messenger of Allah, ﷺ , traveled and stayed for nineteen days, praying two units of prayer at a time. Ibn Abbas said, "When we travel and stay for nineteen days, we pray two units of prayer at a time. And when we stay for longer than that, we pray four units." This is also mentioned in the hadith of Abu Khaythamah and Surayj, that the number is nineteen units.  

البيهقي:٥٤٦٣ – أَخْبَرَنَاهُ مُحَمَّدُ بْنُ عَبْدِ اللهِ الْبِسْطَامِيُّ أنبأ أَبُو بَكْرٍ الْإِسْمَاعِيلِيُّ أَخْبَرَنِي أَبُو يَعْلَى ثنا أَبُو خَيْثَمَةَ وَسُرَيْجُ بْنُ يُونُسَ قَالَا ثنا أَبُو مُعَاوِيَةَ ح وَأَخْبَرَنَا مُحَمَّدٌ أنبأ أَبُو بَكْرٍ أَخْبَرَنِي الْجَوْزِيُّ ثنا مُجَاهِدُ بْنُ مُوسَى وَيَعْقُوبُ الدَّوْرَقِيُّ وَالْفَضْلُ بْنُ الصَّبَّاحِ وَيُوسُفُ قَالُوا ثنا أَبُو مُعَاوِيَةَ وَهَذَا حَدِيثُ الْجَوْزِيِّ عَنْ عَاصِمٍ الْأَحْوَلِ عَنْ عِكْرِمَةَ عَنِ ابْنِ عَبَّاسٍ قَالَ

سَافَرَ رَسُولُ اللهِ ﷺ سَفَرًا فَأَقَامَ تِسْعَةَ عَشَرَ يُصَلِّي رَكْعَتَيْنِ رَكْعَتَيْنِ وَقَالَ ابْنُ عَبَّاسٍ فَنَحْنُ إِذَا سَافَرْنَا فَأَقَمْنَا تِسْعَةَ عَشَرَ صَلَّيْنَا رَكْعَتَيْنِ رَكْعَتَيْنِ وَإِذَا أَقَمْنَا أَكْثَرَ مِنْ ذَلِكَ صَلَّيْنَا أَرْبَعًا وَكَذَلِكَ فِي حَدِيثِ أَبِي خَيْثَمَةَ وَسُرَيْجٍ تِسْعَ عَشْرَةَ