مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2590
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated from Ibn 'Abbas (رضي الله عنه) that The Messenger of Allah (ﷺ) used to let his hair hang down. The mushrikeen used to part their hair and the people of the Book used to let their hair hang down, and he liked to do the same as the people of the Book in matters concerning which he had received no command. Then the Messenger of Allah (ﷺ) parted his hair.
Musnad Ahmad 2605Its isnad is saheeh]