Sahih Bukhari | Volume 5 | Hadith Number 593

Narrated by '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 travellers, But if we stay longer, we offer complete prayers."