Musnad Ahmad 1248:7

Yahya ibn Adam narrated to us, he said: Isra'il narrated to us, from Abu Ishaq, from al-Aswad, from Abu Ma'qal, from Umm Ma'qal, that she asked the Messenger of Allah, peace and blessings of Allah be upon him. He said: "An Umrah in Ramadan is equivalent to a Hajj."