Sahih al-Bukhari — Prophetic Commentary on the Qur'an (Tafseer of the Prophet (pbuh)) — Hadith No: 4917
Translation
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)