Authentic Supplication Number 015.

The Messenger of Allah  said: "Shan't I guide you to a treasure among the treasures of Paradise?", I said: Yes O' Messenger of Allah. He said: "Say: La hawla wa la quwwata 'illa bil-lahi (19)."

Footnotes.
(19) There is neither might, nor strength except by Allah.

Reporters.
Agreed upon by Bukhari and Muslim. Al-Albani included this hadeeth in The Authentic of Good Sayings as #15.