The Prophet ﷺ said, "Whoever does not trim his moustache is not one of us."
حدثنا يحيى عن يوسف بن صهيب ووكيع حدثنا يوسف عن حبيب بن يسار
عن النبي ﷺ قال من لم يأخذ من شاربه فليس منا
Its chain of transmission is sahih. Yusuf bin Suhayb Kindi is trustworthy, and scholars praised him. His hadith appears in the Sunan works, and similarly Habib bin Yasar Kindi. The hadith was narrated by Tirmidhi (5/93, no. 2761) in the chapter on seeking permission, in the section on trimming the moustache, where he said it is hasan sahih, and by Nasai (1/51, no. 13) in the chapter on purification, and by Malik in the chapter on hair (2/947, no. 1).