Musnad Ahmad 1156:208

Mu'adh narrated to us, Ibn 'Awn narrated to us, from al-Hasan, from his mother, from Umm Salamah, who said: I did not forget him on the day of the Trench, while his chest was covered in dust, and he was handing them the bricks, and saying: "O Allah, the true good is the good of the Hereafter, so forgive the Ansar and the Muhajirun." She said: Then Ammar approached, and when he saw him, he said: "Woe to you, O son of Sumayyah, the transgressing party will kill you." He (the narrator) said: So I narrated it to Muhammad, and he said: "From his mother? Verily, she used to enter upon the Mother of the Believers."