The Caliph ‘Umar Ibn Al-Khattaab adviced Sa’d Ibn Abee Waqqaas a second time, this time when he wanted to send him out from Madeenah to Iraaq, to go and fight with the Persians, saying:
“I have appointed you in charge of the war in Iraaq, so heed my advice. You are going to face something difficult, and nothing can save you from it except the truth. Accustom yourself and those who are with you to doing good, start everything with good, and remember that for every good habit there is a means to develop it. The means of developing good is patience. If you are patient in bearing everything that befalls you or happens to you, you will gain the fear of Allaah (or rather, taqwa).
Remember that the fear (or rather, taqwa) of Allaah is a combination of two things: (1) obeying Him, and (2) avoiding disobedience. Those who obey Him do so by hating this world and loving the Hereafter, and those who disobey Him do so by loving this world and hating the Hereafter.
The signs of faith are instilled by Allaah in the heart; some are hidden and some are visible. As for that which is visible, it is when the one who praises a person and the one who criticizes him on the basis of truth are the same to him. As for that which is hidden, it may be manifest in wisdom flowing from his heart on his tongue and people’s love for him.
Do not think little of how important it is for people to love you, for the Prophets asked that people should love them. When Allaah loves a person, He makes others love him too, and when He hates a person, He makes others hate him too.
(Therefore) you should think of your status before Allaah by checking your status before the people who are with you in this cause.”
[Taareekh at-Tabaree]