Musnad Ahmad 32:789

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Humayd, who said: I heard Anas ibn Malik say: The Ansar said: "We are those who pledged allegiance to Muhammad to strive (Jihad) as long as we remain." The Messenger of Allah, peace and blessings of Allah be upon him, answered them: "O Allah, the true good is the good of the Hereafter, so forgive the Ansar and the Muhajirun."