Musnad Abu Ya'la al-Mawsili 151:56

Abu Bakr ibn Abi Shaybah narrated to us, Abu Usamah narrated to us, from Mis'ar, who said: Ma'n ibn Abd al-Rahman brought out a book to me and swore by it that it was his father's handwriting. In it, it said: Abdullah said: "By the One besides Whom there is no god, I have not seen anyone who was more severe against the *mutanatti'in* (those who go to extremes in speech or religious matters) than the Messenger of Allah, may Allah bless him and grant him peace. And I have not seen anyone after him more severe against them than Abu Bakr. And I truly think that Umar was the person on earth who feared them the most."