Musnad Ahmad 27:1529

Ishaq ibn Sulayman narrated to us, he said: I heard Hanzalah mentioning, from Nafi’, from Ibn Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "From the *fitrah* (natural disposition) are: shaving the pubic hair, clipping the nails, and trimming the mustache." And Ishaq said once: "And trimming the mustaches."