(31) باب جَوَازِ الْعُمْرَةِ فِي أَشْهُرِ الْحَجِّ
Ibn Abbas (Allah be pleased with them) reported that Allah's Messenger (ﷺ) observed the morning prayer at Dhu Tawa (a valley near Mecca) and arrived (in Mecca) when four days of Dhul-Hijja had passed and he commanded his Companions that they should change their Ihram (of Hajj) to that of Umra, except those who had brought sacrificial animals with them.
Reference: Sahih Muslim 1240 e
In-book reference: Book 15, Hadith 221
USC-MSA web (English) reference(deprecated numbering scheme): Book 7, Hadith 2862