Musnad Ahmad 26:775

Abu Qatan narrated to us, Shu'bah narrated to us, from Simak, from Ibrahim, from his maternal uncle, from Abdullah ibn Mas'ud, that a man said to the Messenger of Allah (peace and blessings of Allah be upon him): "I met a woman in a garden in Medina, and I obtained from her what is less than sexual intercourse." Then [the following verse] was revealed: "And establish prayer at the two ends of the day and at the approach of the night" [Hud: 114].