From Sahih Muslim (Hadith 2994):
Nafi' thus reported on the authority of Ibn Umar:We entered into the state of Ihram with Allah's Messenger (ﷺ) for Hajj Mufrad and in the narration of Ibn 'Aun (the words are):" Allah's Messenger (ﷺ) entered into the state of Ihram (with the intention) of Hajj Mufrad
Reference: Muslim 2994