Musannaf Ibn Abi Shayba 3:5906

Qasim ibn Malik narrated to us, from Ayyub ibn 'Aidh, from Bukayr ibn al-Akhnas, from Mujahid, from Ibn Abbas, who said: Allah the Exalted 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.