(1) باب مَا جَاءَ فِي التَّقْصِيرِ وَكَمْ يُقِيمُ حَتَّى يَقْصُرَ
Narrated Ibn `Abbas:
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.
Reference: Sahih al-Bukhari 1080
In-book reference: Book 18, Hadith 1
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 2, Book 20, Hadith 186