Musnad Ahmad 626:103

Waki’ narrated to us, Mis’ar narrated to us, from Abu Sakhra, from al-Mughira ibn ‘Abd Allah, from al-Mughira ibn Shu’ba, who said: I spent a night with the Messenger of Allah, peace and blessings of Allah be upon him. He ordered a side of meat, and it was roasted. Then he took a knife and began to cut pieces for me from it. Then Bilal came 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." He said: My mustache had grown long, so he trimmed it for me over a siwak, or he said: "I will trim it for you over a siwak."