سنن الفطرة
Ibn ‘Umar (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "Trim the mustaches and grow the beards."
وعن ابن عمر -رضي الله عنهما- عن النبي -صلى الله عليه وسلم- قال: «أحْفُوا الشَّوَارِبَ وأَعْفُوا اللِّحَى».
شرح الحديث :
Meaning of the Hadīth: A Muslim is required to trim his mustache and not let it untrimmed for over forty days, as reported by Anas (may Allah be pleased with him): "We were given a time limit with regard to trimming the mustache, shaving the pubic hairs, plucking the armpit hairs and clipping the nails. We were not to leave that for more than forty days." In another Hadīth: "Whoever does not trim his mustache is not one of us." So, a Muslim should trim his mustache, either until the whiteness of the skin appears or just removing the part going beyond the lip, which may touch food. As for growing the beard, this includes the hair of the face and on the area of the jawbones. Growing here means letting it grow long and thick and not cutting it at all - as indicated in many Hadīths. The Persians used to cut their beards, and so the Shariah prohibited us from doing so, lest we will resemble them, and Muslims are forbidden from imitating the disbelievers.