Musnad Ahmad 1061:6

Abu Kamil narrated to us, Zuhayr narrated to us, Abdullah ibn Isa narrated to us, Musa ibn Abdullah ibn Yazid narrated to me, from Abu Humayd or Abu Humaydah, who said—and he had seen the Messenger of Allah (may Allah bless him and grant him peace)—he said: The Messenger of Allah (may Allah bless him and grant him peace) said: "If one of you proposes to a woman, there is no blame upon him to look at her, if he is only looking at her for the sake of the proposal, even if she does not know."