Sahih al-Bukhari 1080
Narrated Ibn Abbas :
The Prophet 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 travelled (and stayed) for a longer period we used to offer the full prayer.
حَدَّثَنَا مُوسَى بْنُ إِسْمَاعِيلَ، قَالَ حَدَّثَنَا أَبُو عَوَانَةَ، عَنْ عَاصِمٍ، وَحُصَيْنٍ، عَنْ عِكْرِمَةَ، عَنِ ابْنِ عَبَّاسٍ ـ رضى الله عنهما ـ قَالَ أَقَامَ النَّبِيُّ ﷺ تِسْعَةَ عَشَرَ يَقْصُرُ، فَنَحْنُ إِذَا سَافَرْنَا تِسْعَةَ عَشَرَ قَصَرْنَا، وَإِنْ زِدْنَا أَتْمَمْنَا.
Classification
Sahih (Authentic)
References
Sahih al-Bukhari 1080
Sahih al-Bukhari Vol. 2, Book of Shortening Prayers, Hadith 186