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).
USC-MSA web (English) reference: Vol. 6, Book 60, Hadith 439
Arabic reference: Book 65, Hadith 4917