Ibn ʿAdī said, “In my estimation he was one of those who steal ḥadīth.”
I add: “Nevertheless, Muslim narrated from him, through the following chain: Ibn ʿUyaynah told us; he said: ʿAmr related to us, from ʿIkrimah, from Ibn ʿAbbās who said:

‘Huyayy ibn Akhṭab and Kaʿb ibn al-Ashraf came to Mecca and made a pact with Quraysh to fight the Messenger of Allah ﷺ. The people of Quraysh said to them, “You are the learned ones of old, the People of the Book. Tell us about ourselves and about Muhammad.”
They replied, “What are you and what is Muhammad?”
Quraysh answered, “We slaughter the humped she-camel, give milk mixed with water to drink, ransom captives, provide water for the pilgrims, and keep ties of kinship.”
The two Jews asked, “And what about Muhammad?”
They said, “He is a rootless upstart who has severed our family bonds, and the scroungers of the pilgrims—the Banū Ghifār—have followed him.”
The Jews replied, “No; you are better guided than he and on a straighter path.”
So Allah revealed: ‘Have you not seen those who were given a share of the Scripture, who believe in al-jibt and al-ṭāghūt…’ (Qur’an 4:51).’”

Sufyān said, “Ghifār were thieves in the Age of Ignorance.”

Ibrāhīm ibn Jaʿfar ibn Maḥmūd ibn Maslamah narrated from his father, from Jābir ibn ʿAbd Allāh:
“Kaʿb ibn al-Ashraf stayed in Mecca until he came to Madinah, openly declaring his enmity toward the Prophet ﷺ and lampooning him. The first verses he recited were:

Are you departing, never having gained a noble deed,
And leaving Umm al-Faḍl behind in the Sacred Precinct?

A golden-hued beauty—were she to be wrung, her color would still flow—
Fragrant with fine flasks, with henna and with katam dye.

A daughter of Banū ʿĀmir—my heart is captive to her;
Were she to will it, she could cure Kaʿb of his malady.

…Never had I seen a sun rise before her dawning,
Until she shone upon us in the darkness of the night.”