Abu Huraira (may Allaah be pleased with him) said: the Messenger of Allaah (sallahAllaahu alayhi wasallam) said: “Whoever says when he takes his bed to sleep:
لَا إِلَهَ إِلَّا اللَّهُ وَحْدَهُ لَا شَرِيكَ لَهُ لَهُ الْمُلْكُ وَلَهُ الْحَمْدُ وَهُوَ عَلَى كُلِّ شَيْءٍ قَدِيرٌ ، لَا حَوْلَ وَلَا قُوَّةَ إِلَّا بِاللَّهِ الْعَلِيّ الْعَظِيم ، سُبْحَانَ اللّهِ ، والْحَمْدُللّهِ ، وَ لا اِلهَ اِلَّا اللّهُ ، وَ اللّهُ اَكْبَرُ
‘There is none worthy of worship in truth except Allaah Alone, He has no partners, He has the Dominion and for Him is all perfect praise. He is capable of all things, there is no capability nor is there any might except with Allaah, Far is Allaah from imperfections, and all perfect praise is for Him, there is none worthy of worship in truth except Allaah, and Allaah is the Greatest.
[Whoever says this before sleeping] Allaah forgives his sins –or mistakes (the narrator doubted)- even if they are as numerous as the foam of the sea.” 
I am attaching the audio recitation of this dhikr, in a bid to avoid transliterations as much as possible, so that it can be pronounced properly and so that it can be memorized inshaAllaah.
You will find that in the audio the dhikr is repeated three times, this is for learning purposes; it doesn’t mean you say the dhikr three times before sleeping, I repeated it three times for easier learning inshaAllaah.
Note: This type of dhikr and all other dhikr have to have an effect on the heart, and that we just don’t say them by the tongue, rather we really believe in it; and then proclaim it.
1.Saheeh Targheeb: 607