Musnad Ahmad 32:474

Abu al-Nadr narrated to us, al-Mubarak narrated to us, from Thabit al-Bunani, from Anas ibn Malik, who said: The watering of the camels became difficult for the Ansar, so they gathered at the Prophet, peace and blessings of Allah be upon him, asking him to dig a flowing river for them. The Messenger of Allah, peace and blessings of Allah be upon him, said to them: "Welcome to the Ansar, welcome to the Ansar. By Allah, you will not ask me for anything today except that I will give it to you, and I will not ask Allah for anything for you except that He will grant it to me." Some of them said to others: "Seize the opportunity and ask for forgiveness." So they said: "O Messenger of Allah, supplicate to Allah for our forgiveness." The Messenger of Allah, peace and blessings of Allah be upon him, said: "O Allah, forgive the Ansar, and the children of the Ansar, and the children of the children of the Ansar."