Umar bin Khattab declared at the Treaty of Hudaibiya – I did not entertain any doubt about the prophethood of the Holy Prophet (s.a.w.a.) since I accepted Islam except on that Day of Hudaibiya.
Musannaf v 5 p 332 Sahih Ibn Hibbaan v 11 p 224 Al-Mojam al-Kabir v 20 p 14 Zaad al-Maad Ibn Qayyim v 3 p 257