It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that among the children, after Sayyidah Khadijah (may Allah be pleased with her), the first to perform prayer or accept Islam was Sayyiduna Ali (may Allah be pleased with him).
Hadith Referenceمسند احمد / ومن مسند بني هاشم / 3542
Hadith Gradingحکم دارالسلام:إسناده ضعيف، انظر برقم: 3061