Musnad Ahmad 626:79

Waki’ narrated to us, Mis’ar narrated to us, from Abu Sakhra Jami’ ibn Shaddad, from Mughira ibn ‘Abdullah, from al-Mughira ibn Shu’ba, who said: I was a guest of the Prophet, peace and blessings of Allah be upon him, one night. He ordered a side of meat, and it was roasted. He said: He took the knife and began to cut pieces for me from it. He said: Then Bilal came to him to inform him of the prayer, so he threw down the knife and said: "What is the matter with him? May his hands be covered in dust." Mughira said: My mustache had grown long, so the Messenger of Allah, peace and blessings of Allah be upon him, trimmed it for me over a siwak, or he said: "I will trim it for you over a siwak."