Musnad Ahmad Hadith 13646

It is narrated from Anas (may Allah be pleased with him) that the companions (may Allah be pleased with them) were reciting this couplet while digging the trench: "We are the ones who have certainly pledged allegiance to Islam on the truthful hand of Muhammad (peace be upon him) until death." And the Prophet (peace be upon him) would respond with this phrase: "O Allah! The real goodness is the goodness of the Hereafter, so forgive the Ansar and the Muhajireen." Then barley bread was brought to the Prophet (peace be upon him) on which melted fat was placed. The companions (may Allah be pleased with them) ate from it, and the Prophet (peace be upon him) said: "The real goodness is the goodness of the Hereafter."