Musnad Ahmad 27:1248

'Abd al-Samad narrated to us, Hammam narrated to us, Matar narrated to us, from Salim, from his father, who said: "I traveled with the Prophet, peace and blessings of Allah be upon him, and with Umar, and they would not exceed two rak'ahs. We were astray, then Allah guided us through him, and it is him whom we follow."