Musnad Ahmad 259:1

Yazid ibn Harun narrated to us, he said: Al-Hajjaj ibn Artah informed us, from Muhammad ibn Sulayman ibn Abi Hathma, from Sahl ibn Abi Hathma, who said: I saw Muhammad ibn Maslamah following a woman with his gaze. I said: "You are looking at her, and you are one of the companions of Muhammad, may Allah bless him and grant him peace?" He said: "I heard the Messenger of Allah, may Allah bless him and grant him peace, say: 'If Allah, the Almighty and Majestic, casts into the heart of a man the proposal to a woman, then there is no harm in him looking at her.'"