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, as is the fashion nowadays).
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 5990
Hadith Gradingحکم دارالسلام:حديث صحيح، وهذا إسناد ضعيف، المبارك بن فضالة مدلس وقد عنعن.