Musnad Ahmad 32:2124

'Affan narrated to us, Hammad narrated to us, he said: Thabit informed us, from Anas, that the companions of the Prophet, may God bless him and grant him peace, used to say while they were digging the trench: We are those who pledged allegiance to Muhammad, To Islam, for as long as we remain. And the Prophet, may God bless him and grant him peace, was saying: O God, the true good is the good of the Hereafter, So forgive the Ansar and the Muhajirun. Then barley bread and rancid fat were brought to the Messenger of God, may God bless him and grant him peace, and they ate from it, and the Prophet, may God bless him and grant him peace, said: "The true good is only the good of the Hereafter."