From Sahih Muslim (Hadith 2382):
Abu Huraira reported that there came a person to the Messenger of Allah (ﷺ) and said:Messenger of Allah, which charity is the best? Upon this he said: That you should give charity (in a state when you are) healthy and close-fisted, one haunted by the fear of poverty, hoping to become rich (charity in such a state of health and mind is the best). And you must not defer (charity to such a length) that you are about to die and would he saying: This is for so and so, and this is for so and so. Lo, it has already come into (the possession of so and so)
Reference: Muslim 2382