Musnad Ahmad 27:1737

Ubayd Allah ibn Muhammad al-Taymi narrated to us, Hammad ibn Salamah informed us, from Humayd ibn Yazid Abu al-Khattab, from Nafi’, from Ibn Umar, from the Prophet (peace and blessings of Allah be upon him) that he said: "Whoever drinks wine, then flog him. Then if he drinks it, flog him. Then if he drinks it, flog him." Then he said at the fifth or the fourth time: "Then kill him."