Ibn Shihab (may Allah have mercy on him) narrates from Sahl bin Sa'd (may Allah be pleased with him): I was present with the two who performed li'an in the presence of the Prophet (peace and blessings be upon him), and at that time I was fifteen years old.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب اللعان / 15338