Musnad Ahmad 1248:4

'Abd al-Razzaq narrated to us, he said: Ma'mar informed us, from al-Zuhri, from Abu Bakr ibn 'Abd al-Rahman ibn al-Harith ibn Hisham, from a woman from Banu Asad ibn Khuzaymah who was called Umm Ma'qil, she said: I intended to perform Hajj, but my camel went missing. So I asked the Messenger of Allah, peace and blessings of Allah be upon him, and he said: "Perform 'Umrah in the month of Ramadan, for an 'Umrah in the month of Ramadan is equivalent to a Hajj."