Musnad Ahmad 25:1132

Yahya ibn Hammad narrated to us, Abu 'Awanah narrated to us, from al-A'mash, from Mujahid, from Ibn Abbas, who said: "The Messenger of Allah, may Allah bless him and grant him peace, used to pray while he was in Makkah towards Bayt al-Maqdis (Jerusalem), with the Ka'bah in front of him. And after he migrated to al-Madinah, [he prayed towards it for] sixteen months, then he was turned towards the Ka'bah."