Narrated by:`Ikrima
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.
حَدَّثَنَا أَحْمَدُ بْنُ يُونُسَ، حَدَّثَنَا أَبُو شِهَابٍ، عَنْ عَاصِمٍ، عَنْ عِكْرِمَةَ، عَنِ ابْنِ عَبَّاسٍ، قَالَ أَقَمْنَا مَعَ النَّبِيِّ صلى الله عليه وسلم فِي سَفَرٍ تِسْعَ عَشْرَةَ نَقْصُرُ الصَّلاَةَ. وَقَالَ ابْنُ عَبَّاسٍ وَنَحْنُ نَقْصُرُ مَا بَيْنَنَا وَبَيْنَ تِسْعَ عَشْرَةَ، فَإِذَا زِدْنَا أَتْمَمْنَا.
Bukhari
Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi) • Hadith 332