It is narrated from Sayyiduna Usamah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) was asked about a man who shaved his head before offering the sacrifice. He (peace and blessings be upon him) said: "There is no harm; let him shave."
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 470