Musnad Ahmad 554:2

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