It is narrated from Mughirah bin Shu'bah (may Allah be pleased with him); he said: I became a guest of the household of the Messenger of Allah (peace and blessings be upon him), and the Messenger of Allah (peace and blessings be upon him) cut (meat) for me from a side of a goat with a knife. Then Bilal (may Allah be pleased with him) called the adhan for Fajr, so he (peace and blessings be upon him) put down the knife and said: "What is the matter with him, may his hands be covered in dust?" My moustache had grown long, and he (peace and blessings be upon him) said: "Shall I trim it for you?" So he (peace and blessings be upon him) placed it over a tooth-stick (siwak) and trimmed it.
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 18185