Musannaf Ibn Abi Shayba 3:5905

Waki’ narrated to us, from Abu ‘Awanah, from Bukayr ibn al-Akhnas, from Mujahid, from Ibn Abbas, who said: Allah ordained the prayer of residence as four [rak'ahs], the prayer of travel as two [rak'ahs], and the prayer of fear as one [rak'ah], upon the tongue of His Prophet—or he said: your Prophet, may Allah bless him and grant him peace.