Sayyiduna Ali bin Abi Talib (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "Performing Umrah in Ramadan is equal to a Hajj."
Imam al-Bazzar recorded this narration; its chain contains a narrator, Harb bin Ali, and I have not found anyone who has given his biography; the rest of its narrators are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الحج / 5670