There is nothing wrong with trimming the moustache or shave it altogether

The scholars are
unanimously agreed that it is permissible to trim or cut the moustache,
because of the many ahaadeeth that have been narrated concerning that, such
as the words of the Prophet (peace and blessings of Allaah be upon
him): “Let the beard grow, and trim the moustache.” Narrated by al-Bukhaari
(5442). And he (peace and blessings of Allaah be upon him) said:
“Whoever does not remove anything from the moustache is not one of us.”
Narrated by al-Tirmidhi (2685) and classed as saheeh by al-Albaani in
Saheeh al-Tirmidhi. 

Al-Nawawi (may Allaah have
mercy on him) said in al-Majmoo’ (1/340)” As for cutting the
moustache, there is unanimous agreement that it is Sunnah. End quote. 

But there is a difference
of opinion among the scholars as to how much may be removed. 

Al-Shawkaani said in
Nayl al-Awtaar (1/148): The people differed as to the amount that may be
cut from the moustache. Many of the salaf were of the view that it may be
removed completely and shaved off, because of the apparent meaning of the
words shave and remove completely. This was the view of the Kufis [and is
one view narrated from Imam Ahmad. What is meant by the Kufis is the
followers of Abu Haneefah (may Allaah have mercy on him)]. 

Many others were of the
view that it is not allowed to shave it or remove it completely. This was
the view of Maalik [and of al-Shaafa’i and of Ahmad, according to another
report from him]. 

Imam Maalik was very strict
about shaving the moustache and regarded it as mutilation for which a person
deserved to be disciplined. He said that shaving it was a bid’ah
(innovation) which had appeared among the people. This was narrated from him
by al-Nawawi in al-Majmoo’ and by Ibn al-Qayyim in Zaad al-Ma’aad
and others, but the majority of scholars were of a different view, as they
thought that there was nothing wrong with shaving or cutting it, although
they differed as to which is better. End quote. 

Al-Mardaawi said in
al-Insaaf (1/121): He may shave his moustache or trim the ends, but
shaving is better. This was stated by him [i.e., Imam Ahmad]. End quote.  

Ibn al-Qayyim stated in
Zaad al-Ma’aad (1/171) that Imam Ahmad (may Allaah have mercy on him)
said: If he shaves it there is nothing wrong with it, and if he cuts it
there is nothing wrong with it. The evidence that Imam Ahmad quoted for that
is that the ahaadeeth enjoined shaving and cutting. 

Hence the brother who asked
this question may know that there is nothing wrong with what he is doing,
although it is better to cut the moustache so that the edge of the lip
appears, and not to remove it altogether, because this is what the Messenger
(peace and blessings of Allaah be upon him) used to do. It was narrated
that Ibn ‘Abbaas (may Allaah be pleased with him) said: The Prophet
(peace and blessings of Allaah be upon him) used to cut his moustache. Narrated by Ahmad (2733). Shaykh Ahmad Shaakir said: Its isnaad is saheeh.