"The Prophet performed Umrah four times, one of them was during Rajab."
Jami` at-Tirmidhi Book 9, Hadith 130
← Previous Hadith Ibn Umar was asked: In which month did the Messenger of Allah perform Umrah? He said: 'During Rajab.' So Aishah said: 'The Messenger of Allah did not perform Umrah except that he was with him - meaning Ibn Umar - and he did not ever perform Umrah in the month of Rajab.'"

Jami` at-Tirmidhi Book 9, Hadith 129 Next Hadith → "The Prophet performed Umrah during Dhul-Qa'dah."

Jami` at-Tirmidhi Book 9, Hadith 131