The Prophet salla Allahu alaihi wa sallam said: whoever says this when going to bed, Allah will forgive his sins even if they were as the foam of the sea:
lailah illa Allah wahdahu la shareeka lahu, lahul Mulku walahul hamdu wa howa ala kulli shai’in Qadeer, lahawla wala quwata illah billah al aliyil atheem, subhana Allahi wal hamdu li Allahi, wala ilaha illa Allahu wa Allahu Akbar.