It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Noble Prophet (peace and blessings be upon him) forbade "Qaza’". "Qaza’" means that when cutting a child's hair, some hair is cut and some is left.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 4473
Hadith Gradingحکم دارالسلام:حديث صحيح، م: 2120 عثمان الغطغاني مختلف- وهو متابع.