because of the hadith of the man who used to do this and Allah sent rain on his garden. Once a man heard a voice say, Go and give rain to so-and-so’s garden. The man followed the cloud and reached this man’s garden and asked. I heard a voice saying, Go, go and send rain upon your garden.
What is the good deed? which you do? The man replied. He divides all of his produce in three buckets, uses one for himself and his family. He puts the other back into the business and he spends the third half in the way of Allah.
Islamic Principles Website 