He traced it back to the Prophet ﷺ, who said, "The Beast will come forth and brand the people on their noses. Then they will mingle among you, so much so that a man will buy a camel and be asked, 'From whom did you buy it?' He will reply, 'I bought it from one of the branded ones.'"Yunus, meaning Ibn Muhammad, said, "Then they will mingle among you," without expressing any doubt. He said, "He traced it back."
حدثنا حجين بن المثنى حدثنا عبد العزيز يعني ابن أبي سلمة الماجشون عن عمر بن عبد الرحمن بن عطية بن دلاف المزني لا أعلمه إلا حدثه
يرفعه إلى النبي ﷺ قال تخرج الدابة فتسم الناس على خراطيمهم ثم يغمرون فيكم حتى يشتري الرجل البعير فيقول ممن اشتريته فيقول اشتريته من أحد المخطمين و قال يونس يعني ابن محمد ثم يغمرون فيكم ولم يشك قال فرفعه
Its chain of transmission is sahih. Haythami said (8/6), "Its narrators are the narrators of the Sahih, except for Umar bin Abdul-Rahman bin Atiya, and he is trustworthy. Ibn Hibban also deemed him trustworthy, and Bukhari said nothing about him." The hadith about the beast has come up many times before.