Du'a using Allah's Name Al-Mumeet

اللَّهُمَّ يَا مُمِيتُ قَدِّرْ مَوْتَنَا عَلَى التَّوْحِيدِ وَالْإِسْلَامِ

Allahumma ya Mumeetu qaddir mawtana 'alat tawheedi wal islam

O Allah, The Inflicter of Death, decree our death upon Oneness and Islam.

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