Musnad Ahmad 2:117

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, he said: I heard Yazid ibn Khumayr narrating from Habib ibn ‘Ubayd, from Jubayr ibn Nufayr, from Ibn al-Simt: That he came to a land called Dumin, from Homs, at a distance of eighteen miles, and he prayed two rak‘ahs. I said to him: "Do you pray two rak‘ahs?" He said: "I saw ‘Umar ibn al-Khattab at Dhu al-Hulayfah praying two rak‘ahs, so I asked him, and he said: 'I am only doing as I saw the Messenger of Allah (may Allah bless him and grant him peace) do'—or he said: 'The Messenger of Allah (may Allah bless him and grant him peace) did.'"