Sahih al-Bukhari 1080
Sahih al-Bukhari · Book 18: Shortening the Prayers (At-Taqseer) · For what period of stay one should offer shortened prayers
Narrated Ibn `Abbas:
حَدَّثَنَا مُوسَى بْنُ إِسْمَاعِيلَ ، قَالَ حَدَّثَنَا أَبُو عَوَانَةَ ، عَنْ عَاصِمٍ ، وَحُصَيْنٍ ، عَنْ عِكْرِمَةَ ، عَنِ ابْنِ عَبَّاسٍ ـ رضى الله عنهما ـ قَالَ أَقَامَ النَّبِيُّ صلى الله عليه وسلم تِسْعَةَ عَشَرَ يَقْصُرُ، فَنَحْنُ إِذَا سَافَرْنَا تِسْعَةَ عَشَرَ قَصَرْنَا، وَإِنْ زِدْنَا أَتْمَمْنَا.
The Prophet (ﷺ) once stayed for nineteen days and prayed shortened prayers. So when we travel led (and stayed) for nineteen days, we used to shorten the prayer but if we traveled (and stayed) for a longer period we used to offer the full prayer.