Musannaf Ibn Abi Shayba 19:413

Abu Bakr narrated to us, he said: Abd al-A’la narrated to us, from Sa’id, he said: They reported from Sa’id ibn Abi Dhi’b, from Umar, he said: They are to be separated by the testimony of two or three, and he is to be stoned by the testimony of four or more. If he returns [to the act], he is to be stoned.