Asim bin Kulayb narrates from his father, and he from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both), that the Messenger of Allah (peace and blessings be upon him) commanded the man conducting the li'an to place his hand over the mouth of the one performing li'an at the fifth oath, and to say: "This (oath) makes the punishment binding."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب اللعان / 15343