Musnad Ahmad 27:593

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Abu Farwah al-Hamdani, who said: I heard 'Awn al-Azdi say: 'Umar ibn 'Ubayd Allah ibn Ma'mar was the governor of Fars, so he wrote to Ibn 'Umar asking him about the prayer. Ibn 'Umar wrote back: "Indeed, the Messenger of Allah, may Allah bless him and grant him peace, whenever he departed from his family, he would pray two rak'ahs until he returned to them."