Musnad Ahmad 25:339

Al-Qasim ibn Malik al-Muzani Abu Ja'far narrated to us, from Ayyub ibn 'A'idh, from Bukayr ibn al-Akhnas, from Mujahid, from Ibn Abbas, who said: "Indeed, Allah, the Almighty and Majestic, ordained the prayer upon the tongue of your Prophet, may Allah bless him and grant him peace: four [rak'ahs] while at home, two [rak'ahs] while traveling, and one [rak'ah] during fear."