From Sahih Muslim (Hadith 3012):
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
Reference: Muslim 3012