Tirmidhi

Explanation:
1:
That is, he is opposed to our Sunnah and our way,
because letting the mustache grow is a distinctive mark of the disbelievers and polytheists.

13. Commentary:
➊ The moustache is a sign of puberty; it distinguishes between a child and an adult. However, it is located above the mouth. If it becomes too long, it will touch food and drink, becoming contaminated itself, and food and drink will also enter the stomach along with dust and other impurities. Therefore, trimming the moustache below the upper lip is a rational necessity. This is also the command of Islamic law (Shari‘ah). However, the edges of the moustache that join with the beard may be left untrimmed.
➋ In the aforementioned hadiths (numbers 9 to 11), five natural practices (fitrah) have been mentioned. This does not mean that only these five things are included in fitrah; rather, in other hadiths, some additional matters are also mentioned. For example, in one narration: «عشر من الفطرہ» “Ten things are from fitrah.” (Sahih Muslim, Book of Purification, Hadith: 261) Their mention, insha’Allah, will come at its appropriate place.

(1) Do not trim the moustache, meaning: trimming is only necessary if, for example, they begin to fall into the mouth, become soiled with drink, etc.; otherwise, it is not necessary to trim them every day, nor is it sufficient to trim them just once or twice in a lifetime.

(2) He is not from among us. Meaning: he does not follow our way, or he does not appear to be a Muslim, or the resemblance intended is that he is like non-Muslims. And Allah knows best.