Musnad Ahmad 32:582

'Abd al-Samad narrated to us, 'Ammar—meaning Abu Hashim, the companion of al-Za'farani—informed us, from Anas ibn Malik, that Bilal was late for the morning prayer. The Prophet, peace and blessings of Allah be upon him, said to him: "What detained you?" He said: "I passed by Fatimah while she was grinding [grain], and the child was crying. I said to her: 'If you wish, I will take care of the hand-mill for you, and you take care of the child; or if you wish, I will take care of the child for you, and you take care of the hand-mill.' She said: 'I am more compassionate toward my son than you.' So that is what detained me." He said: "So I showed her mercy; may Allah show you mercy."