Sahih Muslim 2714
Sahih Muslim · Book 48: The Book Pertaining to the Remembrance of Allah, Supplication, Repentance and Seeking Forgiveness · Supplication When Going To Sleep
This hadith has been narrated on the authority of Ubaidullah b. Umar with the same chain of transmitters and he said:
وَحَدَّثَنَا أَبُو كُرَيْبٍ ، حَدَّثَنَا عَبْدَةُ ، عَنْ عُبَيْدِ اللَّهِ بْنِ عُمَرَ ، بِهَذَا الإِسْنَادِ وَقَالَ " ثُمَّ لْيَقُلْ بِاسْمِكَ رَبِّي وَضَعْتُ جَنْبِي فَإِنْ أَحْيَيْتَ نَفْسِي فَارْحَمْهَا " .
Then utter:" My Lord. with Thine name I place my side and if Thou keepest me alive have mercy upon myself"