Musnad Ahmad 30:3208

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