At the end of the Battle of Yarmuk, the King of Rome began to ask his defeated army:
"Shame on you, tell me about those whom you fought.
Are they not men like yourselves?" They said, "Yes O King." He said, "Who was greater in number, you or them?" They said, "We are greater than them in every place."
He said, "Then why have you been defeated?"
An elder from their great heads said, "They have defeated us because they stand to pray at night, they fast during the day, they fulfill their oaths and they are just amongst themselves."
[Ibn Kathir's 'al-Bidayah wal-Nihayah']