From Sahih Muslim (Hadith 2915):
A'isha (Allah be pleased with her) said:We set out with the Messenger of Allah (ﷺ) just at the appearance of the new moon of Dhul- Hijja. We had no other intention but that of performing the Hajj, whereupon the Messenger of Allah (ﷺ) said: He who among you intends to put on Ihram for `Umra should do so for `Umra. The rest of the hadith is the same
Reference: Muslim 2915