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.

Narrated Ibn `Abbas: The Prophet 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.