Musnad Abu Ya'la al-Mawsili 110:41

Shayban ibn Farrukh narrated to us, Abu 'Awanah narrated to us, from 'Asim al-Ahwal and Husayn ibn 'Abd al-Rahman, from 'Ikrimah, from Ibn 'Abbas: That the Prophet (peace be upon him) traveled and stayed for nineteen [days], shortening the prayer. He said: So when we travel and stay for nineteen [days], we shorten the prayer, and if we exceed that, we complete [the prayer].