Sahih Bukhari
Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi)
Volume: 005 / Book Number: 059 / Hadith Number: 593
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