"Pay a great deal of attention to your heart.
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 God comes
in accordance with how much one checks himself.
Ask God to keep you safe and sound,
and frequently repeat the words:
'There is no power and no strength except with God.'
Fear God and put your hope in Him,
and do not take anything for granted..."
(Snippets of Umar ibn al-Khattab's advice to Sa'd Ibn Abi Waqqas [may God be pleased with them both] when he appointed him commander over the Muslim army in Iraq)