Sahih Bukhari Volume 003, Book 027, Hadith Number 009.

Narated By Abu Ishaq : I asked Masruq, 'Ata' and Mujahid (about the 'Umra of Allah's Apostle). They said, "Allah's Apostle had performed 'Umra in Dhi-l-Qa'da before he performed Hajj." I heard Al-Bara' bin 'Azib saying, "Allah's Apostle had performed 'Umra in Dhi-l-Qa'da twice before he performed Hajj."