Musnad Ahmad 435:68

Yahya ibn Sa'id narrated to us, from Ibn Jurayj, who said: Hasan ibn Muslim narrated to me, from Tawus, that Ibn Abbas informed him, That Mu'awiyah informed him, saying: "I shortened [the hair of] the Messenger of Allah, may Allah bless him and grant him peace, with a *mishqas* (a broad-headed arrow/blade)," or he said: "I saw him shortening it with a *mishqas* at al-Marwah."