Surah 2. Al-Baqara, Ayah 195

I.e., "you might bring about your own destruction by withholding your personal and material contribution to this common effort".

Every fight requires the wherewithals for the fight, the "sinews of war". If the war is just and in the cause of God, all who have wealth must spend it freely. That may be their contribution to the Cause, in addition to their personal effort, or if for any reason they are unable to fight. If they hug their wealth, perhaps their own hands are helping in their own self destruction. Or if their wealth is being spent, not in the Cause of God, but in something which pleases their fancy, it may be that the advantage goes to the enemy, and they are by their action helping their own destruction. In all things, their standard should be, not selfishness, but the good of their brethren, for such good is pleasing to God.