Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) says: The first person to accept Islam was Sayyiduna Ali (may Allah be pleased with him).
Al-Tabarani recorded it. Its chain contains Uthman al-Jazari, whom I do not know; the rest of its narrators are the narrators of the Sahih.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب المناقب / 14600
Hadith Gradingمحدثین:إسنادہ ضعیف
Hadith Takhrij«اخرجه الطبراني فى المعجم الكبير (12151 و 10924)»