Musnad Ahmad 3:25

Abdullah narrated to us, Ubaydullah ibn Umar narrated to us, Uthman ibn Umar narrated to us, Imran ibn Hudayr narrated to us, from Abd al-Malik ibn Ubayd, from Humran ibn Aban, from Uthman ibn Affan, that the Prophet, peace and blessings of Allah be upon him, said: "Whoever knows that the prayer is a binding truth, shall enter Paradise."