Volunteers are mentioned in the Quran in these verses:

[2:158] The knolls of Safa and Marwah are among the rites decreed by God. Anyone who observes Hajj or `Umrah commits no error by traversing the distance between them. If one volunteers more righteous works, then God is Appreciative, Omniscient.

[2:184] Specific days (are designated for fasting); if one is ill or traveling, an equal number of other days may be substituted. Those who can fast, but with great difficulty, may substitute feeding one poor person for each day of breaking the fast. If one volunteers (more righteous works), it is better. But fasting is the best for you, if you only knew.

To learn more, read the Quran in English.

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