‘Umar wrote to Sa’d yet again, reminding him of the spiritual means of victory, which were the most important. In this letter he said:
“Pay a great deal of attention to your heart (spiritual well-being). Talk to your troops, exhort them and remind them of the importance of good intentions and of checking on themselves.
Patience, patience, for the help of Allaah comes in accordance with good intentions, and the reward comes in accordance with how much one checks oneself.
Be careful as you are on this mission for His sake, ask Allaah to keep you safe and sound and frequently repeat the words “laa hawla wa laa quwwata illa billaah” (Trans. There is no power nor any strength except with Allaah).
Write and tell me where you have reached with your army, for my lack of knowledge of your situation and that of your enemy is keeping me from writing some of the things I wanted to write. Describe to us the routes of the Muslims and the land that lies between you and al-Madaa’in (the place to where they were heading), so that it is as if I can see it, and give me a clear picture of your situation.
Fear Allaah and put your hope in Him, and do not take anything for granted. Remember that Allaah has made a promise to you. Put your trust in Allaah with regard to the mission you are embarking on, and beware lest you be removed from this position and replaced with someone else.”
[Taareek At-Tabaree, 4/315]