Musnad Ahmad 5:477

Abdullah narrated to us, Uthman ibn Abi Shaybah narrated to me, Muttalib ibn Ziyad narrated to us, from al-Suddi, from Abd Khayr, from Ali, regarding His saying: "You are only a warner, and for every people there is a guide" [Ar-Ra'd: 7]. He said: "The Messenger of Allah, may Allah bless him and grant him peace, is the warner, and the guide is a man from Banu Hashim."