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

وَقَالَ اللَّيْثُ حَدَّثَنِي يُونُسُ، عَنِ ابْنِ شِهَابٍ، أَخْبَرَنِي عَبْدُ اللَّهِ بْنُ ثَعْلَبَةَ بْنِ صُعَيْرٍ، وَكَانَ النَّبِيُّ، ﷺ قَدْ مَسَحَ وَجْهَهُ عَامَ الْفَتْحِ.