Musannaf Ibn Abi Shayba 38:73

Muhammad ibn al-Hasan narrated to us, he said: Sharik narrated to us, from Ishaq, from Abdullah ibn Muhammad, he said: Ali passed by the slain of the people of Basra and said: "O Allah, forgive them." Muhammad ibn Abi Bakr and Ammar ibn Yasir were with him. One of them said to the other: "Do you hear what he is saying?" The other said to him: "Be silent, do not increase him [in anger/agitation]."