Musnad Ahmad 658:121

Ali ibn Ishaq narrated to us, Abdullah ibn al-Mubarak informed us, Sufyan informed us, from Mansur, from Sa’d ibn Ubaydah, and he mentioned it with his chain of narration and its meaning, and he said: "Then perform your ablution as you do for prayer." And he said: "Make them the last of what you speak." He said: So I repeated it to the Prophet, may Allah bless him and grant him peace. When I reached: "I believe in Your Book which You have revealed," I said: "And in Your Messenger." He said: "No, and in Your Prophet whom You have sent."