Allah Is Enough for Us
حَسْبُنَا اللَّهُ وَنِعْمَ الْوَكِيلُ
Hasbunallahu wa ni'mal-wakil.
Allah is enough for us, and He is the best One to rely upon.
What it means
These were the words of the believers when they were warned that an army had gathered against them. The warning was meant to frighten them and it did the opposite; the verse says their faith increased. It is the sentence to reach for when the numbers are against you.
When and how
Say it when you are outnumbered, threatened or waiting on a result you cannot influence.
Its virtue
Ibn Abbas reported that Ibrahim said it when he was thrown into the fire, and Muhammad ﷺ said it when people warned him of a gathered enemy, reported in Sahih al-Bukhari 4563.
Source
Quran 3:173
Quranic text from Tanzil.net, Creative Commons Attribution 3.0.