Mukhtasar Bukhari

Narrated Anas (may Allah be pleased with him): The Messenger of Allah (peace and blessings of Allah be upon him) went towards the trench (which was being dug around Madinah). He saw that the Muhajirun and Ansar were digging the trench early in the cold mornings. They did not have any slaves to do this work for them. When the Prophet (peace and blessings of Allah be upon him) saw their distress and hunger, he recited this verse: "O Allah! The real goodness is the goodness of the Hereafter, so bestow Your blessings upon the Muhajirun and Ansar." In response, the Muhajirun and Ansar said: "We are those who have pledged allegiance to Muhammad (peace and blessings of Allah be upon him) on the condition that we will continue to strive in Jihad as long as there is life in our bodies."