It is narrated from Sayyiduna Buraydah (may Allah be pleased with him) that he said: The first to accept Islam along with the Messenger of Allah (peace and blessings be upon him) were Sayyidah Khadijah (may Allah be pleased with her) and Sayyiduna Ali bin Abi Talib (may Allah be pleased with him).
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19305