Du'a using Allah's Name Al-Mu'min

اللَّهُمَّ يَا مُؤْمِنُ ثَبِّتْ إِيمَانَنَا

Allahumma ya Mu'minu thabbit eemanana

O Allah, The Infuser of Faith, strengthen our faith.

A general supplication, not a narration from the Prophet (peace be upon him).