Sahih Muslim Book 007, Hadith Number 2884.
Chapter : Excellence of performing Umra in the month of Ramadhan.
Ataa reported: I heard Ibn Abbas (Allah be pleased with him) narrating to us that Allah's Messenger (may peace be upon him) said to a woman of the Ansar (Ibn Abbas had mentioned her name but I have forgotten it):'What has prevented you that you do not perform Hajj along with us? She said: We have only two camels for carrying water. One of the camels has been taken by my husband and my son for performing Hajj and one has been left for us for carrying water, whereupon he (the Holy Prophet) said: So when the month of Ramadan come, perform Umra, for 'Umra in this (month) is equal to Hajj (in reward).