Hadith -- Zakat

Allah (The Glorified and the Exalted) has strongly warned those who are negligent in the fulfillment of these duties. Thus He said, "Those who hoard up treasures of gold and silver and spend them not in the way of Allah; give them the news of a painful punishment, on the Day when that (wealth) will be heated in the Fire of Hell and with it will be branded their forehead, their sides, and their backs, (and it will be said to them:) 'This is the treasure which you hoarded for yourselves. Now taste of what you used to hoard.'" 
[Al-Quran 9:34-35]

And also: "And let not those who hoard up that which Allah hath bestowed upon them of His bounty think that it is better for them. Rather, it is worse for them. That which they hoard will be their collar on the Day of Resurrection. Allah's is the heritage of the heavens and the earth, and Allah is Informed of what you do." [Al-Quran 3:180]

Further, At-Tabarani relates, on the authority of 'Ali, that the Prophet (PBUH) said: "Allah has enjoined upon rich Muslims a due to be taken from their properties corresponding to the needs of the poor among them. The poor will never suffer from starvation or lack of clothes unless the rich neglect their due. If they do, Allah will surely hold them accountable and punish them severely."
(related by At-Tabarani in Al-'Aswat and as-Saghir)

Narrated Abu Huraira :
Allah's Apostle said, "Whoever is made wealthy by Allah and does not pay the Zakat of his wealth, then on the Day of Resurrection his wealth will be made like a bald-headed poisonous male snake with two black spots over the eyes. The snake will encircle his neck and bite his cheeks and say, 'I am your wealth, I am your treasure.'"
(Bukhari, Vol. 2, Book 24, No.486)