Fiqh-us-Sunnah Volume 5, Hajj and ‘Umrah, Fiqh 5.121B.

A person may perform ‘Umrah during the months of Hajj, without performing Hajj. ‘Umar performed ‘Umrah during the month of Shawwal and returned to Madinah without performing Hajj. One may, like ‘Umar, perform ‘Umrah even before Hajj.

Taw’us said, “In the Days of Ignorance the Arabs considered performing an ‘Umrah during the months of Hajj as one of the major sins. And they used to say, “When (the month of) Safar ends, and the camels are back, and wounds in their legs are healed, and when traces of footprints (of pilgrims) are effaced, then one wishing to perform ‘Umrah may do so.”

After the advent of Islam, the people were commanded to perform ‘Umrah even during the months of Hajj. Thus, the ‘Umrah was permitted during the months of Hajj till the Day of Judgement.