It is narrated from Sayyidah Umm Ma'qil (may Allah be pleased with her) that she had made it binding on herself to perform Hajj with the Prophet (peace and blessings be upon him), but it did not become possible for her. So she asked the Prophet (peace and blessings be upon him): "What will suffice for me?" He (peace and blessings be upon him) said: "Umrah in Ramadan."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 21830