On the authority of Ibn 'Umar (radhiyallaahu 'anhumaa), the Messenger of Allaah (sallallaahu 'alayhe wa sallam) said:
"Whoever goes to sleep in a state of purity,
an angel will stand guard over him.
He will not wake up at any time of the night,
except that the angel says,
‘O Allaah forgive So-and-son, your servant -
for verily he went to sleep in a state of purity!’ "
This report was collected by 'Abdullaah ibn Mubaarak in his Hadeeth (2/101/2), Ibn Hibbaan (#167), and others. Al-Albaanee authenticated it in Silsilatul-Ahaadeethis-Saheehah (#2539).