The Women - 4:95
Those of the believers who sit in their places without a valid excuse are not equal to those who fight in the Way of Allah with their wealth and their lives. Allah has made those who fight with their wealth and their lives superior in rank to those who sit. Allah promises them all good things. However, Allah has made those who fight superior to those who sit with a greater reward;
A valid excuse for staying behind in battle:
The Victory/48:16
"Say to the remnant of the Arabs, "You will be called upon against a powerful people. You will fight them until they surrender. If you obey, Allah will give you a good reward. If you turn away, as you turned away before, He will punish you with a painful torment."
The Victory/48:17
There is no hardship for the blind, no hardship for the paralyzed, and no hardship for the sick. Whoever follows Allah and His messenger, He will admit him into Gardens from which rivers flow. And whoever turns away, He will punish him severely.
The Repentance/9:90
Those of the Arabs who fabricated excuses came to you for permission. So those who denied Allah and His messenger sat down. A painful torment will touch the disbelievers.
The Repentance/9:91
There is no condemnation for the weak, the sick and those who have nothing to give for help, provided they are sincerely devoted to Allah and His Messenger. Those who do good will not be condemned. Allah is Forgiving, Merciful.
The Repentance/9:92
"Likewise, those who come to you to give them a ride and you say, "I cannot find a vehicle to give you a ride,"and they turn back with tears in their eyes because of their financial inability."
The Repentance/9:93
Only those who asked you for permission while they were rich are reprehensible. They asked to be with those who stayed behind, and Allah stamped their hearts. Therefore they do not know.
The Repentance/9:94
"When you return to them (from the war), they offer you excuses. Say: "Do not make excuses in vain; we do not believe in you any more, Allah has informed us of your situation. Allah and His messenger will see your affairs, and then you will be returned to the One Who knows the secret and the open. He will inform you of everything you do."