Sahih Muslim Book 7, Hadith Number 2861.

Chapter : Permissibility of Umra during the mouth of Hajj.

Ibn ‘Abbas (Allah be pleased with them) reported: Allah’s Messenger (may peace be upon him) 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.