Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates: The first to accept Islam was Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him).
Al-Tabarani recorded it in al-Mu'jam al-Awsat. It contains more than one da'if (weak) narrator.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب المناقب / 14311