Musnad Ahmad 2:126

Hashim ibn al-Qasim narrated to us, Shu'bah narrated to us, from Yazid ibn Khumayr al-Hamdani, Abu 'Amr, who said: I heard Habib ibn 'Ubayd narrating from Jubayr ibn Nufayr, from Ibn al-Samt: That he went out with 'Umar to Dhu al-Hulayfah, and he prayed two rak'ahs. So I asked him about that, and he said: "I only do as I saw the Messenger of Allah, may Allah bless him and grant him peace, do."