The Messenger of Allah ﷺ said, "Trim the moustaches and let the beards grow."
حدثنا يحيى عن عبيد الله أنبأنا نافع
قال رسول الله ﷺ أحفوا الشوارب وأعفوا اللحى
Its isnad is sahih. Malik related it in Muwatta 3:123 from Abu Bakr bin Nafi, from his father Nafi, in similar wording, but Malik did not hear it from Nafi himself, he heard it from his son Abu Bakr. Abu Dawud related it in 4:135 through Malik's route, and Munziri said, Muslim and Nasai also transmitted it.Trimming the mustaches means going to great lengths in clipping them. Letting the beards grow means letting their hair grow abundant and not trimming it as one does the mustaches, from the word afa meaning something becoming plentiful and increasing, one says afaytuhu and afaituhu. So said Ibn Athir.