Musnad Ahmad 27:791

Waki' narrated to us, Sa'id ibn al-Sa'ib narrated to us, from Dawud ibn Abi 'Asim, who said: I asked Ibn 'Umar about the prayer at Mina. He said: "Have you heard of Muhammad, may Allah bless him and grant him peace?" I said: "Yes, and I have believed in him." He said: "For he used to pray two rak'ahs at Mina."