Musnad Ahmad 32:1032

Isma'il narrated to us, Yahya ibn Abi Ishaq informed us, he said: I asked Anas ibn Malik about shortening the prayer, and he said: "We traveled with the Prophet, peace and blessings of Allah be upon him, from Medina to Mecca, and he prayed two rak'ahs with us until we returned." I asked him: "Did he stay?" He said: "Yes, we stayed in Mecca for ten [days]."