From Sahih Bukhari (Hadith 4917):
Narrated Ibn `Abbas:(regarding the Verse):-- 'Cruel after all that, base-born (of illegitimate birth).' (68.13) It was revealed in connection with a man from Quaraish who had a notable sign (Zanamah) similar to the notable sign which usually-hung on the neck of a sheep (to recognize it)
Reference: Bukhari 4917