There are many ingrained beliefs and references to generosity in the various religious texts.
Bible - ‘It is more blessed to give than to receive.’
Torah - 8 levels of charity, one of the highest being "give to the poor without knowing to whom one gives, and without the recipient knowing from who he received"
Buddhists practice giving (dana) - True dana arises from the intention underlying the act of giving. It is not that you are supposed to have only pure motives but rather that your intention is to cultivate purity of generosity without self-consideration.
But Religion aside, it just feels right to be generous, you don't actually need anyone to tell you that it is a good thing to practice. This is a big subject and can be over-complicated, but to cut through that a nice summary is...
Benefits of generosity to your children:
How can children be taught to practice generosity?
The hack .. Be a role-model: give freely, using these as a few ways to show how generous you can be - all without expecting anything in return.
The most important act of generosity
When children see you acting out of generosity, they will quickly catch on. When you know they have been generous, ask them how it felt afterwards. Ask them how it might have felt for the recipient. Would it have felt different for either of them if they did not know each other? How did it feel not wanting or needing anything in return?
Always on the lookout for the ultimate Kid Hacks