From Sahih Bukhari (Hadith 3187):
Narrated Anas:The Prophet (ﷺ) said, ''Every betrayer will have a flag on the Day of Resurrection" One of the two subnarrators said that the flag would be fixed, and the other said that it would be shown on the Day of Resurrection, so that the betrayer might be recognized by it
Reference: Bukhari 3187