Mujahid narrates: I was sitting with Abdullah bin Umar (may Allah be pleased with them both) together with Urwah bin al-Zubayr, and Urwah asked him: In which month did the Messenger of Allah (peace and blessings be upon him) use to perform Umrah? He said: "He used to perform Umrah in Rajab."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13469