Musnad Ahmad 25:380

'Ali ibn 'Asim narrated to us, from Dawud ibn Abi Hind, from 'Ikrimah, from Ibn 'Abbas: That a man from the Ansar apostatized from Islam and joined the polytheists. Then Allah the Exalted revealed: "How shall Allah guide a people who disbelieved after their belief" [Al 'Imran: 86] to the end of the verse. His people sent it to him, so he returned repentant. The Prophet, peace and blessings of Allah be upon him, accepted that from him and let him go.