Musannaf Ibn Abi Shayba 36:244

Zayd ibn al-Hubab narrated to us, he said: Musa ibn Ubaydah narrated to us, he said: Abdullah ibn Ubaydah informed me, from Abu Salih, the freed slave of Umm Hani, that al-Harith ibn Suwayd pledged allegiance to the Messenger of Allah (may Allah bless him and grant him peace) and believed in him. Then he joined the people of Makkah and witnessed Uhud, and he fought against the Muslims. Then he felt remorse (literally: it fell into his hand), so he returned to Makkah. He wrote to his brother, Julas ibn Suwayd: "O my brother, I have regretted what I did, so I repent to Allah and return to Islam. Mention that to the Messenger of Allah (may Allah bless him and grant him peace). If you have hope for a repentance for me, then write to me." He mentioned it to the Messenger of Allah (may Allah bless him and grant him peace), and Allah revealed: "How shall Allah guide a people who disbelieved after their belief?" He said: A group of his companions said, "He is one who enjoys himself, then returns to Islam." Then Allah revealed: "Indeed, those who disbelieve after their belief and then increase in disbelief, never will their repentance be accepted, and they are the ones astray."