From Sahih Bukhari (Hadith 1695):
Narrated Al-Miswar bin Makhrama and Marwan:The Prophet (ﷺ) set out from Medina with over one thousand of his companions (at the time of the Treaty of Hudaibiya) and when they reached Dhul-Hulaifa, the Prophet (ﷺ) garlanded his Hadi and marked it and assumed Ihram for `Umra
Reference: Bukhari 1695