Sahih Bukhari
Wills and Testaments (Wasaayaa)
Volume: 004 / Book Number: 051 / Hadith Number: 11
Narrated Abu Huraira:
A man asked the Prophet, "O Allah's Apostle! What kind of charity is the best?" He replied. "To give in charity when you are healthy and greedy hoping to be wealthy and afraid of becoming poor. Don't delay giving in charity till the time when you are on the death bed when you say, 'Give so much to so-and-so and so much to so-and so,' and at that time the property is not yours but it belongs to so-and-so (i.e. your inheritors)."