Musnad Ahmad 30:2077

Yahya ibn Ghaylan narrated to us, he said: Al-Mufaddal narrated to us, he said: Ubayd Allah ibn Zahr narrated to me, that Abu Hurayrah said: "O people, indeed Allah, the Almighty and Majestic, has obligated upon you, by the tongue of your Prophet, the prayer: four [rak'ahs] while at home, and two rak'ahs while traveling."