At-Tawbah · 9:44

44. It is not the characteristic of those who truly have faith in Allah and the Day of Judgement to ask the Messenger (peace be upon him)for permission to stay behind from striving for Allah’s sake with their wealth and their selves. Rather, it is their characteristic to rally when they are called to fight, and to strive with their wealth and their persons. Allah knows those among His servants who are Mindful, who do not ask to be excused except for genuine reasons that really prevent them from going with the Messenger.