‘Umar’s[RA] amazing letter to Sád Ibn Abi Waqqas[RA]
“To Sa’d Ibn Abi Waqqas — Commander of the Muslim Army in conquest of Persia”… Then after, I order you and all troops that are along with you to be obedient to Allah in all circumstances … As this (being obedient to Allah) is better than the weapons against the enemy and a strong stratagem (device) in the war.
I order you and the soldiers who are with you to be more cautious and afraid of your own crimes and sins (and do not commit them) than your enemy, as the crimes and sins of the soldiers are MORE DANGEROUS TO THEM THAN THEIR ENEMY.
The Muslims are victorious only because their enemies are disobedient to Allah, and had it not been so, we have no power over them, because neither our number is equal to their number, nor are our weapons like theirs.
If we commit crimes and sins as they do, then they (our enemies) will have superiority over us in power and we will not gain victory over them. WE DO NOT OVERPOWER THEM WITH OUR STRENGTH.
And you should also know that in this marching of yours (for Allah’s cause), there are guards (angels) upon you from Allah (to watch you) and they all know what you do.
Companies: So be shy from them and do not commit Allah’s disobedience (crimes and sins) while you are going in Allah’s cause and do not say: “Our enemy is worse than us, so they will not overpower us!”
Perhaps some people who are worse than the others may overpower the others as the (disbelievers) Magians… Overpowered the Children of Israel when they (the latter) involved themselves with Allah’s disobedience (crimes and sins). So they (disbelievers, Magians) entered the very innermost parts of their homes and it was a promise (completely) fulfilled.
And ask Allah the assistance over your ownselves, (to save you from the crimes and sins) just as you ask Allah for the victory over your enemies … I ask Allah for that both for you and for us”.
[revised edition of Sahih Al Bukhari in the section preparing for jihad written by Shiekh Abdullah bin Muhammed bin Humaid]