Sayyiduna Abu Hurayrah (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) was asked: Which charity is best? He (peace and blessings be upon him) said: "You will certainly be informed of it: that you give charity while you are healthy and covetous of it, wishing to keep it and fearing poverty. So do not wait until your soul reaches your throat and then you say: 'So much is for so-and-so, and so much is for so-and-so.' Beware! It has by then already become so-and-so's." (Imam Muslim narrated it in his Sahih via Zuhayr bin Harb, and Imam Bukhari narrated it through other chains.)
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7832