Sadaqah (charity) is the concept of giving charity voluntarily fi sabillilah فِي سَبِيلِ ٱللَّٰهِ (for the cause of Allah).
Sadaqah also describes a voluntary charitable act towards another being, whether through generosity, love, compassion or faith. These acts are not necessarily physical or monetary. Simple good deeds such as a smile, or a helping hand, are seen as acts of Sadaqah.
A man came to the Prophet (ﷺ) and said: “O Messenger of Allāh which Sadaqah is the one that gives the greatest amount of reward from Allāh (سبحانه و تعالى)? The Prophet (ﷺ) said: “That you give in charity whilst you are in good health and you are feeling miserly and you want to covet that wealth that you possess, and you fear poverty for yourself and that you hope for riches.” (Al-Bukhāri: no. 1419)