From Sahih Bukhari (Hadith 4299):
Narrated `Ikrima:Ibn `Abbas said, "We stayed for 19 days with the 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: Bukhari 4299