Musnad Ahmad 30:644

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from Suhayl ibn Abi Salih, from his father, from Abu Hurayrah, that the Prophet (peace and blessings of Allah be upon him) said: "Whoever drinks wine, then flog him. Then if he drinks, then flog him. Then if he drinks, then flog him. Then if he drinks the fourth time, then kill him."