‘Umar[RA] writes to Abu ‘Ubaidah[RA]
On August 22, 634 (22nd Jamadi-ul-Akhir, 13 Hijri), Abu Bakr died and Umar became Caliph. On the same day the new Caliph issued his first order: Khalid was dismissed from the command of the Muslim army in Syria! He wrote to Abu Ubaidah as follows:
In the name of Allah the Beneficent, the Merciful.
I urge upon you the fear of Allah who lives eternally while everything else perishes; who has guided us away from wrongdoing and taken us out of darkness into light.
I appoint you commander of the army of Khalid bin Al Waleed. So take charge as is your duty.
Send not the Muslims to their destruction for the sake of plunder; and place not the Muslims in a camp without reconnoitring it and knowing what is there.
Send not expeditions except in properly organised units. And beware of taking any steps which may lead to the annihilation of the Muslims.
Allah has tried me with you and tried you with me. Guard against the temptations of this world lest they destroy you as they have destroyed others before you; and you have seen how they felt.