Musnad Ahmad 25:1473

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