Musnad Ahmad 32:1283

'Abd al-Samad narrated to us, 'Abd Allah ibn Abi Yazid narrated to us, he said: I heard Musa ibn Anas narrating from his father, that the water-drawing camels became difficult for the Ansar, so they came to the Prophet, peace and blessings of Allah be upon him, for him to supplicate for them or to dig a canal for them. The Prophet, peace and blessings of Allah be upon him, was informed of that, so he said: "They shall not ask me for anything today except that I will grant it to them." The Ansar were informed of that, and when they heard what the Prophet, peace and blessings of Allah be upon him, said, they said: "Supplicate to Allah for our forgiveness." He said: "O Allah, forgive the Ansar, and the children of the Ansar, and the children of the children of the Ansar."