Musnad Ahmad 27:1307

'Affan narrated to us, Hammam narrated to us, Matar narrated to us, from Salim ibn 'Abdullah ibn 'Umar, from his father, who said: "We traveled with the Messenger of Allah (ﷺ) and with 'Umar, and I did not see them exceed two rak'ahs. We were astray, then Allah guided us through him, and it is him whom we follow."