Musnad Ahmad 30:2198

Muhammad ibn Ja'far narrated to us, Mu'ammar narrated to us, he said: Al-Zuhri informed us, from Sa'id ibn al-Musayyib, from Abu Hurairah, he said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "Five are of the *fitrah* (natural disposition): circumcision, shaving the pubic hair, plucking the underarm hair, clipping the nails, and trimming the mustache."