1. It is permissible to shave the moustache or to trim it. However to trim the moustache in the manner that the hair is removed and the skin above the upper lip becomes visible is better than shaving. The moustache should not be left to the point that it overlaps the upper lip.
عن ابن عباس رضي الله عنهما قال: كان النبي صلى الله عليه وسلم يقص أو يأخذ من شاربه وكان إبراهيم خليل الرحمن صلوات الرحمن عليه يفعله. (ترمذي رقم 2760)
Hadhrat ibn Abbaas (Radhiyallahu Anhuma) reports that Hadhrat Rasulullah (Sallallahu Alaihi Wasallam) used to trim his moustache, and Hadhrat Ibraaheem (Alayhis Salaam), (who had received the title of) Khalilur Rahmaan [the friend of Allah Ta’ala], also used to trim his moustache.”