Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: The Messenger of Allah (peace and blessings be upon him) said: "Whoever fasts one day of Ramadan seeking reward, he will receive for his fast such a reward that, if all the people of the world from its beginning until its end were gathered together, it would provide them with an abundance of food and drink, and he would not ask the people of Paradise for anything of it."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11153