Musnad Ahmad 27:1475

Ishaq narrated to us, Malik informed us, from Abdullah ibn Dinar, from Ibn Umar, who said: While the people were at Quba in the morning prayer, a comer came to them and said: "Indeed, the Quran has been revealed to the Messenger of Allah (peace and blessings of Allah be upon him) tonight, and he has been commanded to face the Kaaba, so face it." Their faces had been toward Ash-Sham (the Levant), so they turned around toward the Kaaba.