Ata bin Abi Rabah (may Allah have mercy on him) narrates from Sayyiduna Ibn Abbas and Sayyiduna Ibn al-Zubayr (may Allah be pleased with them) that the Prophet (peace and blessings be upon him) said: "Performing Umrah in Ramadan is equal to a Hajj."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13868