The polytheists used to part their hair, while the People of the Book let their hair hang down. Yaqub added, "their hair." The Messenger of Allah ﷺ loved and was pleased to align with the People of the Book. Yaqub said, "in matters concerning which he had not been commanded." Ishaq said, "in what he had not been commanded about." So he let his forelock hang down, then later he parted it.
حدثنا إسحاق بن عيسى حدثني إبراهيم يعني ابن سعد عن الزهري ويعقوب حدثني أبي عن ابن شهاب عن عبيد الله بن عبد الله
كان المشركون يفرقون رءوسهم وكان أهل الكتاب يسدلون قال يعقوب أشعارهم وكان رسول الله ﷺ يحب ويعجبه موافقة أهل الكتاب قال يعقوب في بعض ما لم يؤمر قال إسحاق فيما لم يؤمر فيه فسدل ناصيته ثم فرق بعد
Its chain is sahih. In H, it reads "Ibrahim, meaning Ibn Saad," which is an error we have corrected from K.As for the statement of Abdullah bin Ahmad, "my father said, and Yaqub," this means that his father, the Imam, said, "Ishaq narrated to us," then said, "and Yaqub." So this is a narration from Ishaq bin Isa and from Yaqub bin Ibrahim bin Saad, both of them from Ibrahim bin Saad, from Zuhri.In H, it reads "Ibn Yaqub said" instead of "my father said, and Yaqub," which is an error we have corrected from K.The hadith was narrated by the two Shaykhs and the compilers of the Sunan works, as noted in Awn al-Mabud 4:131–132.