Authentic Supplication Number 018.

Abdullah bin Khubayb said: we went out on a rainy and very dark night, seeking the Prophet  to pray for us, then we found him, and he said: "Qul (say)". So I said nothing, then he said: So I said nothing, then he said: "Qul." I said: O' Messenger of Allah! What should I say? He said "Qul huwal-lahu 'ahad" and the Mu`awwathatayn (34) when you enter upon evening time and when you enter upon morning time, three times, He [Allah] will protect you from everything".

Footnotes.
(34) Qul huwal-lahu 'ahad and the Mu`aw-wathatayn are the last three suwar (plural for surah) of the Qur'an. Refer to a translation of the meaning of the Qur'an for an explanation of these suwar.

Reporters.
Reported by Abu Dawood, An-Nisa'ee, and At-Tirmithi who said it was comely-sound. Al-Albani agreed with him, said it was of good ascription and included it in The Authentic of Good Sayings as #18.