Under Umar, 'Amr became the Governor of Palestine in 63X C.E. In 639 C.E. after the death of Ubaidullah b Jarah, 'Amr became the supreme commander of the armed forces in Syria. That provided him an opportunity to conquer Egypt. Umar was averse to the extension of Muslim dominions. When 'Amr b Al 'Aas pressed for the conquest of Egypt, Umar reluctantly gave his permission to try his luck in Egypt. 'Amr b Al 'Aas invaded Egypt with a small force in the closing months of 639 C.E. Egypt fell before the Muslim arms as if by a miracle.
"When a person dies, his works end, except for three: ongoing charity, knowledge that is benefited from, and a righteous child who prays for him."
Prophet Mohammed (PBUH)
"The best of what a man leaves behind are three: a righteous child who supplicates for him, ongoing charity the reward of which reaches him, and knowledge that is acted upon after him."
Sunan Ibn Mājah
"Every day two angels come down from Heaven and one of them says, 'O Allah! Compensate every person who spends in Your Cause,' and the other (angel) says, 'O Allah! Destroy every miser.'"
Sahih Bukhari