Sahih Bukhari | Volume 4 | Hadith Number 758

Narrated by Ibn 'Abbas
Allah's Apostle used to let his hair hang down while the infidels used to part their hair. The people of the Scriptures were used to letting their hair hang down and Allah's Apostle liked to follow the people of the Scriptures in the matters about which he was not instructed otherwise. Then Allah's Apostle parted his hair.