From Sahih Muslim (Hadith 2383):
Abu Huraira reported that a person came to the Messenger of Allah (ﷺ) and said:Messenger of Allah, which charity is the greatest in reward? (The Prophet said): By your father, beware, you should give charity (in a state when you are) healthy and close-fisted, haunted by the fear of poverty, and still hoping to live (as rich). And you must not defer charity (to the time) when you are about to die, and would then say:" This is for so and so, and this for so and so." It has already become the possession of so and so
Reference: Muslim 2383