"So Abu Bakr decided to fight them, then 'Umar said: 'O Abu Bakr, how can you fight the people when the Messenger of Allah , I will fight them for withholding it.' 'Umar said: 'By Allah, as soon as I realized that Allah has expanded the chest of Abu Bakr to fight them, I knew that it was the truth.'"
Sunan an-Nasa'i Book 37, Hadith 10