Musnad Ahmad 32:1733

Muhammad ibn Yazid narrated to us, Sadaqah, the owner of the flour, narrated to us, from Abu Imran al-Jawni, 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: forty days."