Sayyiduna al-Bara' bin Azib (may Allah be pleased with them both) said: The Messenger of Allah (peace and blessings be upon him) performed three Umrahs, all three of them in Dhu al-Qa'dah. Sayyidah Aishah (may Allah be pleased with her) said: He knows that he (peace and blessings be upon him) performed four Umrahs, including the Umrah that he performed with his Hajj.
This has also been narrated in the hadith of Sayyiduna Jabir bin Abdullah (may Allah be pleased with them both), but that is not authentic.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 8840