Chapter: It is permissible to perform 'Umrah during the months of Hajj
Ibn 'Abbas (Allah be pleased with them) reported: Allah's Messenger ( صلی اللہ علیہ وسلم ) came along with his Companions when four days had passed out of ten days (of Dhu'l-Hijja) and they were pronouncing Talbiya for Hajj, and he (the Holy Prophet) commanded them to change (this Ihram) into that of 'Umra.
Authentic