Musnad Ahmad 27:1377

Isma'il ibn 'Umar narrated to us, Sufyan narrated to us, from 'Abdullah ibn Dinar, from Ibn 'Umar, who said: "While the people were praying in the Mosque of Quba', a man came and said: 'Indeed, the Quran has been revealed to the Messenger of Allah (may Allah bless him and grant him peace), and he has been commanded to face the Ka'bah.'" He said: "So they turned around."