It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the 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 / 5175
Hadith Gradingحکم دارالسلام:إسناده صحيح ، م : 2120 .