Musnad Ahmad 32:1170

Yazid ibn Harun narrated to us, Hammam informed us, from Anas ibn Sirin, who said: We met Anas ibn Malik when he arrived from Ash-Sham, and we met him at 'Ayn al-Tamr while he was praying upon his mount facing other than the Qibla. So we said to him: "Indeed, you are praying toward other than the Qibla." He said: "Were it not that I saw the Messenger of Allah, may Allah bless him and grant him peace, doing that, I would not have done it."