Musnad Abu Ya'la al-Mawsili 110:19

Khalaf ibn Hisham al-Bazzar, Abu Abd al-Rahman al-Allaf, and others narrated to us, they said: Abu Awanah narrated to us, from Bukayr ibn al-Akhnas, from Mujahid, from Ibn Abbas, who said: Allah obligated the prayer upon the tongue of your Prophet, may Allah bless him and grant him peace, as four [rak'ahs] while at home, two rak'ahs while traveling, and one rak'ah during fear.