It is narrated from Sayyiduna Sahl bin Sa'd al-Sa'idi (may Allah be pleased with him) that the husband who performed li'an (mutual imprecation) divorced her with three divorces right there 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) let it take effect; and what was done in the presence of the Messenger of Allah (peace and blessings be upon him) became the practice. Sahl (may Allah be pleased with him) said: "I was present at this in the presence of the Messenger of Allah (peace and blessings be upon him). After that, the practice concerning the couple who perform li'an became established that they are separated, and then they may never come together again." Ibn Shihab said: "At that time the man said: 'What an evil servant of Allah I would be if I lied in the presence of the Messenger of Allah (peace and blessings be upon him) and took upon myself the burden of a slander.'"
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5663