It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both): "The Messenger of Allah (peace and blessings be upon him) (at first) used to let the hair over his forehead hang down, as the People of the Book used to let it hang down; then after that he (peace and blessings be upon him) began to part (his hair), as the Arabs used to part it. And it was the practice of the Messenger of Allah (peace and blessings be upon him) that when he was uncertain about a matter that both the polytheists and the People of the Book used to do, provided that no revelation had yet come about it, he (peace and blessings be upon him) would act according to the way of the People of the Book."
No one narrated this hadith from Abdullah bin Dinar al-Bahrani (may Allah have mercy on him) except Isma'il (may Allah have mercy on him), and no one from Isma'il (may Allah have mercy on him) except Adam (may Allah have mercy on him), and Muhammad (may Allah have mercy on him) is alone in narrating it.
Hadith Referenceالمعجم الأوسط / باب الباء / 3110
Hadith Takhrij«أخرجه البخاري فى «صحيحه» برقم: 3558، 3944، 5917، ومسلم فى «صحيحه» برقم: 2336، وابن حبان فى «صحيحه» برقم: 5485، والنسائي فى «المجتبى» برقم: 5253 / 1، والنسائي فى «السنن الكبرى» برقم: 9282، وأبو داود فى «سننه» برقم: 4188، وابن ماجه فى «سننه» برقم: 3632، وأحمد فى «مسنده» برقم: 2244، 2400، 2648، 2990، وأبو يعلى فى «مسنده» برقم: 2377، 2554، وابن أبي شيبة فى «مصنفه» برقم: 25581، والطحاوي فى «شرح معاني الآثار» برقم: 2805، 2806، والطحاوي فى «شرح مشكل الآثار» برقم: 3357، 3358، 3634، 3635، والترمذي فى «الشمائل» برقم: 30»