It is narrated from Sayyiduna Asim bin Adi (may Allah be pleased with him) that he was with the Messenger of Allah (peace and blessings be upon him) when this verse was revealed: "Then they do not bring four witnesses" [Surah al-Nur: 4]. I said: O Messenger of Allah! By the time they bring four witnesses, the traitor will already have fulfilled his desire. (He said:) He (peace and blessings be upon him) had not yet even stood up when his cousin (who was his father's brother) came with his wife, who was carrying a child and saying: He is yours, while he was saying: He is not mine. He said: Then the verse of li'an (mutual imprecation) was revealed. So I was the first to speak about it, and I was the one put to the test by it.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14532