(52) باب مَقَامُ النَّبِيِّ صلى الله عليه وسلم بِمَكَّةَ زَمَنَ الْفَتْحِ
Narrated `Ikrima:
Ibn `Abbas said, "We stayed for 19 days with Prophet on a journey during which we used to offer shortened prayers." Ibn `Abbas added, "We offer the Qasr prayer (i.e. shortened prayer) If we stay up to 19 days as travelers, But if we stay longer, we offer complete prayers.
Reference: Sahih al-Bukhari 4299
In-book reference: Book 64, Hadith 332
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 5, Book 59, Hadith 593