It is narrated from Mughirah bin Shu'bah (may Allah be pleased with him); he said: One night I became a guest of the Messenger of Allah (peace and blessings be upon him), and he (peace and blessings be upon him) ordered a side of ribs (meat), which was roasted. Then he (peace and blessings be upon him) took up the knife and began cutting (meat) from it (the ribs) with it. Then Bilal (may Allah be pleased with him) came to inform him of the prayer, 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?" He said: My moustache had grown long, so he (peace and blessings be upon him) placed it over a tooth-stick (siwak) and trimmed it.
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 18183