Musnad Ahmad 31:816

Ya'qub narrated to us, my father narrated to us, from Ibn Ishaq, who said: Abdullah bin Abd al-Rahman bin Ma'mar bin Hazm narrated to me, from Sulayman bin Muhammad bin Ka'b bin 'Ujrah, from his paternal aunt Zaynab bint Ka'b—and she was the wife of Abu Sa'id al-Khudri—from Abu Sa'id al-Khudri, who said: The people complained about Ali. He said: So the Messenger of Allah, peace and blessings of Allah be upon him, stood among us as a preacher, and I heard him say: "O people, do not complain about Ali, for by Allah, he is indeed rough/strict regarding the sake of Allah, or in the path of Allah."