Some argue trimming the moustache is obligatory as the Prophet (saw) said, "Whoever does not trim his moustache is not one of us."
The problem with such a claim is the Prophet (saw) often mentions the phrase "not one of us" in ahadith on matters which are not obligatory. This phrase in and of itself does not imply obligatory - further evidence is required for such a conclusion. Some examples of ahadith where such a phrase is used include:
- من لم يحلق عانته، ويقلم أظفاره، ويجز شاربه، فليس منا
"Whoever does not shave his pubic hair, cut his nails and trim his moustache is not one of us." (al-Suyuti, al-Jami' as-Sa'ir)
- لْوِتْرُ حَقٌّ فَمَنْ لَمْ يُوتِرْ فَلَيْسَ مِنَّا
"Whoever does not perform witr prayer is not one of us." (Sunan Abi Dawud 1419)
- ثَلاثَةٌ عَلَيَّ فَرِيضَةٌ وَهِيَ سُنَّةٌ لَكُمْ : الْوِتْرُ ، وَالسِّوَاكُ ، وَقِيَامُ اللَّيْلِ "
"Three acts are wajib for me and sunnah (desirable for you: witr, siwak (using the tooth stick) and night vigil." (al-Bayhaqi, al-Sunan al-Kuba, Vol. 7, p. 62)
- لنِّكَاحُ مِنْ سُنَّتِي ، فَمَنْ لَمْ يَعْمَلْ بِسُنَّتِي فَلَيْسَ مِنِّي
"Marriage is from my sunnah, whoever dies not follow my sunnah is not one of me." (ibn Majah. Book 9, Hadith 1919)
Ahadith have to be scrutinised further when the phrase "he is not one of us" is used as it may entail various ahkam. So when the Prophet (saw) said, "He is not one of us who calls for (or boasts about) race" asabiyya is prohibited - it is not because of the phrase "not one of us" is used, but the prohibition appears explicitly in many other evidences.
Likewise the prophet(saw) said, "He is not one of us who cheats us." Cheating is forbidden, again not because of the phrase "not one of us," but due to other evidences prohibiting cheating in an explicit manner..
As for the hadith, "Whoever does not trim his moustache is not one of us," given the absence of any evidences suggesting prohibition, this narration then can be understood as trimming is desirable. When asked about the trimming of the moustache, ibn Hanbal said, "It is from the Sunnah".