Musnad Ahmad 27:402

Yazid narrated to us, Abd al-Aziz ibn Abd Allah ibn Abi Salamah narrated to us, from Umar ibn Husayn, from Abd Allah ibn Abi Salamah, from Abd Allah ibn Abd Allah ibn Umar, from Abd Allah ibn Umar, who said: "We were with the Messenger of Allah, may Allah bless him and grant him peace, on the morning of Arafah. Among us were those who were saying the Takbir (Allahu Akbar) and among us were those who were saying the Tahlil (La ilaha illa Allah). As for us, we were saying the Takbir." He [the narrator] said: I said [to him]: "I am amazed at you! How did you not ask him how the Messenger of Allah, may Allah bless him and grant him peace, acted?"