The Prophet [SAW] saw a boy, part of whose head had been shaven and part had been left. He forbade that and said: "Shave all of it, or leave all of it."  

أَنَّ النَّبِيَّ ﷺ رَأَى صَبِيًّا حَلَقَ بَعْضَ رَأْسِهِ وَتَرَكَ بَعْضَ فَنَهَى عَنْ ذَلِكَ وَقَالَ احْلِقُوهُ كُلَّهُ أَوِ اتْرُكُوهُ كُلَّهُ