Sunan an-Nasa'i 5057
It was narrated that Al-Hasan and Muhammad said:
"Combing one's hair (should be done) every other day."
أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا بِشْرٌ، عَنْ يُونُسَ، عَنِ الْحَسَنِ، وَمُحَمَّدٍ، قَالاَ التَّرَجُّلُ غِبٌّ ‏.‏