Sayyiduna Sahl bin Sa'd (may Allah be pleased with him) narrates that he pronounced three divorces in the presence of the Messenger of Allah (peace and blessings be upon him), and the Messenger of Allah (peace and blessings be upon him) enforced them. What was done in the presence of the Messenger of Allah (peace and blessings be upon him) was made a sunnah. Sayyiduna Sahl (may Allah be pleased with him) says: I was present with the Messenger of Allah (peace and blessings be upon him) at that time; afterwards this sunnah was established for those who perform li'an: they are to be separated, and then they will never come together again. [Sahih. It has preceded.]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب اللعان / 15322