Musnad Abu Ya'la al-Mawsili 117:50

Hadbata narrated to us, Hammad bin Salamah narrated to us, from Thabit, from Anas, that the companions of the Messenger of Allah (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, to fight as long as we remain." And the Prophet (may Allah bless him and grant him peace) was saying: "O Allah, the true life is the life of the Hereafter, so forgive the Ansar and the Muhajirun."