Sahih Bukhari Volume 003, Book 027, Hadith Number 010.

Narated By Ata : I heard Ibn 'Abbas saying, "Allah's Apostle asked an Ansari woman (Ibn 'Abbas named her but 'Ata' forgot her name), 'What prevented you from performing Hajj with us?' She replied, 'We have a camel and the father of so-and-so and his son (i.e. her husband and her son) rode it and left one camel for us to use for irrigation.' He said (to her), 'Perform 'Umra when Ramadan comes, for 'Umra in Ramadan is equal to Hajj (in reward),' or said something similar."