Musnad Ahmad 937:219

'Ali ibn Ishaq narrated to us, he said: 'Abdullah said: Yunus narrated to me, from al-Zuhri, he said: I heard Abu al-Ahwas, the freed slave of Banu Layth, narrating to us in the gathering of Ibn al-Musayyib, and Ibn al-Musayyib was sitting, that he heard Abu Dharr saying: The Messenger of Allah (may Allah bless him and grant him peace) said: "Allah, the Almighty and Majestic, continues to be attentive to the servant in his prayer as long as he does not turn away. But if he turns his face away, He turns away from him."