Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates this saying of the Prophet (peace and blessings be upon him): "When it is the seventh day after the child's birth, sacrifice an animal on his behalf, remove the harmful thing (that is, the hair of the head) from him, and give him a name."
Al-Tabarani recorded it in al-Awsat and al-Kabir, and its narrators are thiqat (trustworthy).