Sayyiduna Abu Rafi' (may Allah be pleased with him) narrates that he gave the Prophet (peace and blessings be upon him) the good news of Sayyiduna Abbas (may Allah be pleased with him) accepting Islam, so the Prophet (peace and blessings be upon him) set him free.
Al-Tabarani recorded it in al-Mu'jam al-Awsat, and its chain is hasan (good).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب المناقب / 15469