Sahih Bukhari Volume 4, Book 52, Hadith Number 87.

Narated By Anas : Allah’s Apostle went towards the Khandaq (i.e. Trench) and saw the Emigrants and the Ansar digging in a very cold morning as they did not have slaves to do that for them. When he noticed their fatigue and hunger he said, “O Allah! The real life is that of the Here-after, (so please) forgive the Ansar and the Emigrants.” In its reply the Emigrants and the Ansar said, “We are those who have given a pledge of allegiance to Muhammad that we will carry on Jihad as long as we live.”