In the morning Allah's Messenger (PbUH) asked me, "What did your prisoner do."
I replied, "O Allah's Messenger (PbUH)! He complained of his great need and of
too many dependents, so I took pity on him and set him free." Allah's
Apostle said, "Verily, he told you a lie and he will return." I waited
for him attentively for the third time, and when he (came and) started
stealing handfuls of the foodstuff, I caught hold of him and said, "I
will surely take you to Allah's Messenger (PbUH) as it is the third time you
promise not to return, yet you break your promise and come." He said,
"(Forgive me and) I will teach you some words with which Allah will
benefit you." I asked, "What are they?" He replied, "Whenever you go
to bed, recite "Ayat-al-Kursi"-- 'Allahu la ilaha illa huwa-l-Haiy-ul
Qaiyum' till you finish the whole verse. (If you do so), Allah will
appoint a guard for you who will stay with you and no satan will come
near you till morning. " So, I released him. In the morning, Allah's
Apostle asked, "What did your prisoner do yesterday?" I replied, "He
claimed that he would teach me some words by which Allah will benefit
me, so I let him go." Allah's Messenger (PbUH) asked, "What are they?" I
replied, "He said to me, 'Whenever you go to bed, recite Ayat-al-Kursi
from the beginning to the end ---- Allahu la ilaha illa
huwa-lHaiy-ul-Qaiyum----.' He further said to me, '(If you do so),
Allah will appoint a guard for you who will stay with you, and no
satan will come near you till morning.' (Abu Huraira or another
sub-narrator) added that they (the companions) were very keen to do
good deeds. The Prophet (PbUH) said, "He really spoke the truth, although he
is an absolute liar. Do you know whom you were talking to, these three
nights, O Abu Huraira?" Abu Huraira said, "No." He said, "It was
Satan."