Musnad Ahmad 1248:1

Rawh and Muhammad ibn Mus'ab narrated to us, they said: Al-Awza'i narrated to us, from Yahya ibn Abi Kathir, from Abu Salamah ibn 'Abd al-Rahman, from Umm Ma'qil al-Asadiyyah, that she said: "O Messenger of Allah, I intend to perform Hajj, but my camel is lean, so what do you command me?" He said: "Perform 'Umrah in Ramadan, for an 'Umrah in Ramadan is equivalent to a Hajj."