Abdullah bin Abbas reported,

When Kaab bin al-Ashraf came to Mecca, the Quraysh said to him, "Are you the best of the people of Medina and their chief?"He replied, "Yes."They said, "Do you not see this man cut off from his people, who claims to be better than us, while we are the people of the pilgrimage and the custodians of the Kaaba?"He said, "You are better than him."So the verse was revealed, "Indeed, your enemy is the one cut off" [Kawthar 108:3], and there was also revealed, "Have you not seen those who were given a portion of the Scripture, who believe in superstition and false objects of worship" up to His statement, "you will never find for him a helper" [Nisa 4:51–52].

عن ابن عباس قال

لما قدم كعب بن الأشرف مكة قالت له قريش: أنت خير أهل المدينة وسيدهم؟ قال: نعم، قالوا: ألاترى إلى هذا المنبتر من قومه، يزعم أنه خير منا، ونحن أهل الحجيج وأهل السدانة، قال: أنتم خير منه، فنزلت: {إن شانئك هو الأبتر}، ونزلت: {ألم تر إلى الذين أوتوا نصيبا من الكتب يؤمنون بالجبت والطاغوت} إلى قوله: {فلن تجد له نصيرا}[النساء: ٥١-٥٢].