سنن النسائي

It was narrated that Abu Hurairah said: "A man came to the Prophet (ﷺ) and said: 'O Messenger of Allah, what kind of charity brings the greatest reward?' He said: 'To give in charity when you are healthy and feeling miserly, and fearing poverty and hoping for a long life. Do not wait until the (death rattle) reaches the throat and then say: "This is for so and so," and it nearly became the property of so and so (the heirs).'"