He traced it back to the Prophet ﷺ, who said, "Every pact made during the Jahiliyyah, Islam only increased it in firmness and strength."
حدثنا حجاج أخبرنا شريك عن سماك عن عكرمة
رفعه إلى النبي ﷺ قال كل حلف كان في الجاهلية لم يزده الإسلام إلا شدة أو حدة
Its chain of transmission is sahih. It is found in Majma al-Zawaid 8:173, with an addition from Abu Yaala, who said, and from Ibn Abbas, who said, the Messenger of Allah ﷺ said, "There is no alliance in Islam, and whatever existed in it during the pre-Islamic era, Islam has only made it stronger or more intense." Abu Yaala and Ahmad narrated it in a shortened form, and the men of their chains are the men of sahih narrations. He means this hadith, whose meaning has already passed as a mursal narration from Zuhri, along with a hadith of Abdul-Rahman bin Awf, 1655.