Musnad Ahmad 33:678

Zakariya ibn 'Adi narrated to us, 'Ubayd Allah—meaning Ibn 'Amr al-Raqqi—informed us, from 'Abd al-Karim, from 'Ata', from Jabir, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "An 'Umrah in Ramadan is equivalent to a Hajj."