Surah 2. Al-Baqara, Ayah 245

I.e., by sacrificing one's life in, or devoting it to, His cause.

Spending in the cause of God is called metaphorically "a beautiful loan". It is excellent in many ways: (1) it shows a beautiful spirit of self-denial; (2) in other loans there may be a doubt as to the safety of your capital or any return thereon; here you give in the Lord of All, in Whose hands are the keys of want or plenty; giving you may have manifold blessings, and withholding you may even lose what you have. If we remember that our goal is God, can we turn away from His cause?