It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that once the Prophet (peace and blessings be upon him) saw a child whose some hair had been cut and some had been left. The Prophet (peace and blessings be upon him) forbade this and said: "Either shave all the hair of the head or leave all of it."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 5615