Authentic Supplication Number 026.

It is reported by way of Abi Hurayrata , that someone used to come upon him and take from the charity - which the Prophet  had made him guardian over - night after night, so on the third night, he said: I will report you to the Messenger of Allah , the man said: Let me teach you some phrases with which Allah will cause you benefit - they were very eager for good - so he said: When you go to bed, read 'Aayaht 'al-kursee (47) completely, for there is still upon you from Allah a guardian, and no shaytan can get close to you until you enter upon morning. Then he said: "He told you the truth and he is a liar, ([that was a shaytan])".

Footnotes
(47) Verse 244 of Surat Al-Baqarah (#2).

Reporters.
Reported by Al-Bukhari, and Al-Albani included it in The Authentic of Good Sayings as #26.