The Messenger of Allah (صلى الله عليه و سلم) said, "How can I feel comfort while the angel responsible for The Trumpet has placed The Trumpet to his mouth and lowered his forehead, awaiting when he will be commanded (by Allah to blow in it)?"

They said, "O Allah's Messenger, what should we say?''

He said, "Say: Allah is sufficient for us, and He is Al-Wakeel (the best disposer of affairs); We have placed our trust in Allah." (hasbunAllaha wa ni'mal Wakeel, 'ala Allahi tawakalnaa)

The Companions repeated this invocation, saying, "Allah is sufficient for us, and He is Al-Wakeel (the best disposer of affairs); We have placed our trust in Allah." [Sunan at-Tirmidhi, Saheeh according to Shaykh Albani]

Another example of this du'aa is the last ayah of Surah Tawbah:

Transliteration: HasbiyAllahu, la ilaha il Huwa, 'alayhi tawakaltu, wa Huwa Rabb al-'arsh al-'atheem.

Translation: Allah is sufficient for me. I have placed my trust in Him, He is the Lord of the Majestic Throne. [Surah Tawbah, verse 129]

What is the significance of saying this du'a from surah Tawbah?

The Messenger of Allah (صلى الله عليه و سلم) said, "Allah will grant whoever recites this seven times in the morning and evening whatever he desires from this world or the next." [Saheeh, Sunan Abee Dawood]