Musnad Ahmad 30:3598

Sulayman ibn Dawud narrated to us, Abu 'Awanah informed us, from 'Umar ibn Abi Salamah, from his father, from Abu Hurayrah, that the Prophet, peace and blessings of Allah be upon him, said: "If he drinks wine, then whip him. If he returns [to it], then whip him." Then he said regarding the fourth [time]: "Then kill him."