Sahih Bukhari Volume 4, Book 52, Hadith Number 88.

Narated By Anas : The Emigrants and the Ansar started digging the trench around Medina carrying the earth on their backs and saying, “We are those who have given a pledge of allegiance to Muhammad that we will I carry on Jihad as long as we live.” The Prophet kept on replying, “O Allah, there is no good except the good of the Hereafter; so confer Your Blessings on the Ansar and the Emigrants.”