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).
Sahih al-Bukhari, Vol. 6, Book of Prophetic Commentary on the Qur'an, Hadith 439
Sahih al-Bukhari, Book of Prophetic Commentary on the Qur'an, Hadith 439