Musnad Ahmad 32:1702

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