Musnad Ahmad 25:187

Yahya narrated to us, from Ibn Jurayj, who said: 'Ata' informed us, saying: I heard Ibn Abbas say: The Messenger of Allah (peace and blessings of Allah be upon him) said to a woman from the Ansar—Ibn Abbas named her, but I forgot her name—: "What prevented you from performing Hajj with us this year?" She said: O Prophet of Allah, we only had two water-carrying camels. The father of so-and-so and his son—referring to her husband and her son—rode one camel, and they left one camel for us to irrigate with. The Prophet (peace and blessings of Allah be upon him) said: "So when Ramadan comes, perform Umrah during it, for an Umrah in it is equivalent to a Hajj."