Sayyiduna Abdullah bin Abbas (may Allah be pleased with him) said: A man of the Ansar apostatised and joined the polytheists, so Allah revealed this verse: "How shall Allah guide a people who disbelieved after their belief and had witnessed that the Messenger is true and clear proofs had come to them? And Allah does not guide the wrongdoing people." [Surah Al Imran: 86]. His people wrote this to him, and he said: By Allah! Neither have my people lied, nor has the Prophet (peace and blessings be upon him) lied against Allah, and Allah is the most truthful of the three. So he came back and repented, and the Prophet (peace and blessings be upon him) pardoned him.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المرتد / 16830