Musnad Ahmad 1206:1

Yahya ibn Sa'id narrated to us, from Hisham, who said: Yahya ibn Abi Kathir narrated to me, from Abu Salamah, from Ma'qil ibn Umm Ma'qil, from Umm Ma'qil al-Asadiyyah, who said: My mother intended to perform Hajj, but her camel was lean. She mentioned that to the Prophet, peace and blessings of Allah be upon him, and he said: "Perform Umrah in Ramadan, for an Umrah in Ramadan is like a Hajj."