"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."  

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  

“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)   

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  

[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.  

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