Chapter 16: The Book of Pilgrimage
Ibn 'Abbas (Allah be pleased with them) reported: Allah' s Messenger (ﷺ) 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.
Sahih Muslim 1240d