Musnad Ahmad 32:1168

Yazid ibn Harun narrated to us, Sadaqah ibn Musa informed us, Abu Imran al-Jawni narrated to us, from Anas ibn Malik, who said: "The Messenger of Allah, may Allah bless him and grant him peace, set a time limit for us for trimming the mustache, clipping the nails, and shaving the pubic hair: once every forty days."