Mishkat al-Masabih 30 Fine Qualities كتاب المناقب 144

وَعَنْ سَعْدٍ قَالَ: رَأَيْتُنِي وَأَنَا ثَالِثُ الْإِسْلَامِ وَمَا أَسْلَمَ أَحَدٌ إِلَّا فِي الْيَوْمِ الَّذِي أَسْلَمْتُ فِيهِ وَلَقَدْ مَكَثْتُ سَبْعَةَ أَيَّامٍ وَإِنِّي لثالث الْإِسْلَام. رَوَاهُ البُخَارِيّ

Sa’d said: I have seen myself as. the third member of Islam, no one having accepted it except on the day I did, and during the space of seven days I constituted a third of Islam[*]. *The claim is that after Abu Bakr and Khadija no one accepted Islam before he did, and that no one else accepted it for a week after he did. Bukhari transmitted it, and Baghawi rendered it in his Mu’jam .