It is narrated from Sayyidah Umm Ma'qil (may Allah be pleased with her) that she asked the Messenger of Allah (peace and blessings be upon him), saying: "I had prepared for Hajj, but an obstacle came up." The Prophet (peace and blessings be upon him) said to her: "Perform Umrah in Ramadan, for an Umrah in it is like a Hajj."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 21831