Musnad Ahmad 1034:87

Abu Sa'id, the freed slave of Banu Hashim, narrated to us, Sulayman ibn Bilal narrated to us, from 'Amr ibn Abi 'Amr, from 'Abdullah ibn 'Abd al-Rahman, one of the Banu 'Abd al-Ashhal, from Hudhayfah, that the Prophet, peace and blessings of Allah be upon him, said: "By the One in Whose hand is my soul, you shall certainly enjoin the good and you shall certainly forbid the evil, or else He will surely send upon you a people, then you will call upon Him, and it will not be answered for you."