Spending in God's way (infāq) and charity (ṣadaqa) are treated in detail in the Qur'an — especially 2:261-274.
Elements in the Qur'an
- Multiplied return: "Those who spend their wealth in God's way are like a grain that grows seven ears, in each ear a hundred grains." (2:261)
- How to give: without reproach or injury (2:262-264), not for show (2:264), from the good of what you earn (2:267).
- Openly or secretly: "If you give charity openly it is good; but if you conceal it and give it to the poor, that is better for you." (2:271)
- The measure: "You will not attain piety (birr) until you spend from what you love." (3:92) Asked what to spend: "the surplus." (2:219)
- To whom: parents, kin, orphans, the needy and the traveller (2:215).
Ṣadaqa is not zakāh
Zakāh is the defined, commanded due (see Zakāh); ṣadaqa/infāq covers every voluntary good. The Qur'an often names both with the same root (ṣ-d-q); in 9:60 and 9:103 "ṣadaqa" includes zakāh.
An honest limit
The Qur'an gives the spirit and manners of giving; the amount is mostly left to one's means ("the surplus", 2:219) — no fixed rate is imposed.
Source: Qur'anic verses (M. Okuyan meal).