Is trimming the moustache sunnah or obligatory?
Question

How authentic is this hadith? “It was narrated that Zaid bin Al-Arqam said:
I heard the Messenger of Allah say: ‘Whoever does not take from his mustache, he is not one of us.’”?
2) If it is authentic, does this mean that trimming your mustache is fard?
3) There is a hadith that says “Do the opposite of what the polytheists do ; let the beard grow long and clip the moustache.” From this hadith, scholars said that keeping the beard is fard, so why is cutting the mustache a sunnah when it is mentioned in the same hadith? Why did scholars say that part of the hadith is an obligation whereas the other part is a sunnah, arent both of them commands from the prophet?
Praise be to Allah.Trimming the moustache is one of the proven sunnahs of the fitrah (sound human inclination).
Ibn al-Mundhir (may Allah have mercy on him) said:
The reports that are soundly narrated from the Messenger of Allah (blessings and peace of Allah be upon him) indicate that trimming the moustache and nails is part of the fitrah (sound human inclination), and that he enjoined trimming the moustache and letting the beard grow.
End quote from al-Awsat, 1/238.
But the scholars differed to the degree to which it is prescribed: is it a recommended sunnah or is it an obligatory action?
Some of the scholars were of the view that it is obligatory, and this is what is implied by the apparent meaning of the two hadiths referred to.
Ibn Muflih (may Allah have mercy on him) said:
and he should trim his moustache… Ibn Hazm narrated that there was consensus that trimming the moustache and letting the beard growth is obligatory.
Our companions and others said that it is recommended, and the Prophet (blessings and peace of Allah be upon him) instructed us to do that, as he said: “And be different from the polytheists.” (al-Bukhari and Muslim). Muslim narrated that he said: “Be different from the Zoroastrians.” It was narrated from Zayd ibn Arqam in a marfu‘ report: “Whoever does not remove anything from his moustache is not one of us.” Narrated by Ahmad, an-Nasa’i and at-Tirmidhi and classed as authentic by the latter. This version, according to our companions, indicates that it is prohibited.
End quote from al-Furu‘, 1/151-152.
Ibn al-Qayyim (may Allah have mercy on him) said:
As for trimming the moustache, the evidence indicates that it is obligatory to trim it when it becomes long, and this is the correct view, because the Messenger of Allah (blessings and peace of Allah be upon him) enjoined that, and because he (blessings and peace of Allah be upon him) said: “Whoever does not remove anything from his moustache is not one of us.”
End quote from Tuhfat al-Mawdud, p. 257.
Shaykh Muhammad ibn ‘Ali Adam al-Ethiopi (may Allah have mercy on him) said:
Ibn Hazm (may Allah have mercy on him) quoted this hadith, the hadith “and trim the moustache,” and similar evidence for it being obligatory to trim the moustache, as noted in al-Fat-h. In al-Manhal he added: Some of the Hanafis and the majority of scholars are of the view that it is recommended. Then he said: What is more likely to be correct is the first view. And Allah knows best.
End quote from Dhakhirat al-‘Uqba, 1/380.
The majority of scholars are of the view that it is sunnah, and they think that the two hadiths referred to do not say that it is obligatory to remove something from the moustache by cutting or shaving.
With regard to the hadith of Zayd ibn Arqam, according to which the Messenger of Allah (blessings and peace of Allah be upon him) said: “Whoever does not remove anything from his moustache is not one of us,” it was narrated by at-Tirmidhi, 2761, and others. A number of scholars stated that it is authentic, including at-Tirmidhi who said, after narrating the hadith: This is an authentic, sound hadith.
They thought that what was meant by the phrase “is not one of us” is that he is not following our guidance and path.
Al-‘Iraqi (may Allah have mercy on him) said:
The hadith of a man from Banu Ghifar, “Whoever does not shave his pubic hair, clip his nails and trim his moustache is not one of us,” indicates that doing these things is obligatory.
The response to that is twofold:
Firstly, this hadith is not proven, because its isnad includes Ibn Luhay`ah, and the scholarly reservations about him are well known. Rather only the part about trimming the moustache is proven, as was narrated and classed authentic by at-Tirmidhi, and was also narrated by an-Nasa’i, from Zayd ibn Arqam, who said: I heard the Messenger of Allah (blessings and peace of Allah be upon him) say: “Whoever does not remove anything from his moustache is not one of us.”
Secondly, what is meant – assuming that it is soundly narrated – is that he is not following our path, because the Prophet (blessings and peace of Allah be upon him) said: “He is not one of us who does not recite the Qur’an in a melodious voice.” This is definitely the intended meaning, and Allah knows best.
End quote from Tarh at-Tathrib, 2/82.
Regarding the hadith of Ibn ‘Umar, according to which the Prophet (blessings and peace of Allah be upon him) said: “Be different from the polytheists: let your beard grow and trim your moustaches” (al-Bukhari, 5829; Muslim, 259),
they thought that the view that it is obligatory to let the beard grow and that it is prohibited to shave it does not necessarily imply that it is obligatory to cut the moustache and is prohibited to leave it, because the fact that these things are mentioned together in this command does not mean that they are equal in the degree of the command, as is the view of the majority of scholars of usul al-fiqh.
Az-Zarkashi (may Allah have mercy on him) said:
With regard to the two phrases being mentioned together, it does not mean that they are equal to other rulings that are not mentioned, in contrast to the view of Abu Yusuf and al-Muzani.
Commentary: the fact that the two things are mentioned together  with regard to a ruling does not mean that they are equal to other rulings that are not mentioned. Hence what is obligatory may be mentioned after something that is recommended, as in the verse in which Allah, may He be exalted, says (interpretation of the meaning):
{Eat of [each of] its fruit when it yields and give its due [zakah] on the day of its harvest}
[al-An`am 6:141].
Abu Yusuf, among the Hanafis, and al-Muzani among us, said: This means that they are equal, because the fact that one is mentioned after the other means that the ruling applies equally to both of them.
End quote from Tashnif al-Masami`, 2/757.
The commentator Shaykh Muhammad al-Amin ash-Shinqiti (may Allah have mercy on him) said:
The view that when two things are mentioned together they are subject to the same ruling is regarded as weak by most of the scholars of usul…
But some of the scholars regarded quoting that as evidence as being sound.
End quote from Adwa’ al-Bayan, 3/266.
Based on that, each phrase has its own ruling, depending on the context and other evidence.
Regarding the phrase “trim your moustaches”, it was said that what is meant is to keep the moustache neat and clean, and it is more appropriate for that to be recommended, as is the case with using the siwak and putting on perfume before going to Jumu`ah prayer, and the like.
Ibn Hajar (may Allah have mercy on him) said:
Al-Qadi Abu Bakr ibn al-‘Arabi went too far when he said: In my view, the five characteristics mentioned in this hadith are all obligatory, and if a person neglects one of them, he will no longer look like a human, let alone look like a Muslim.
He said something similar in Sharh al-Muwatta’, and Abu Shamah commented on it by noting that the things that are required to make one look neat and clean do not need a command from the Lawgiver, because it is sufficient that one has a natural inclination to do them, so merely recommending them is enough.
End quote from Fat-h al-Bari, 10/339-340.
And Allah knows best.