Source: Tafsir al-Ahlam al-Kabir (Ibn Sirin)
The mustache in a dream indicates wealth. Its length indicates drinking intoxicants or falling into debt. Trimming it indicates following the Sunnah and removal of worry.
Source: Ta'tir al-Anam fi Ta'bir al-Manam (Al-Nabulsi)
The mustache indicates a servant or a chamberlain. Whoever sees his mustache is long will fall into worry.
Source: Al-Isharat fi Ilm al-Ibarat (Ibn Shahin)
The mustache is interpreted as wealth. Whoever sees himself trimming it is spending his money in obedience.