Musnad Ahmad 32:1008

Ubaydah narrated to us, from Humayd al-Tawil, from Anas ibn Malik, who said: The Prophet, peace and blessings of Allah be upon him, went out on a cold or chilly morning, and behold, the Emigrants (Muhajirun) and the Helpers (Ansar) were digging the trench. He said: "O Allah, indeed the good is the good of the Hereafter, so forgive the Helpers and the Emigrants." They answered him: We are those who pledged allegiance to Muhammad, To strive (Jihad) as long as we remain, forever.