Sahih Bukhari, Chapter: Obligatory Charity Tax (Zakat)

Once I went to him (the Prophet ) and he said, "By Allah in Whose Hands my life is (or probably said, 'By Allah, except Whom none has the right to be worshipped) whoever had camels or cows or sheep and did not pay their Zakat, those animals will be brought on the Day of Resurrection far bigger and fatter than before and they will tread him under their hooves, and will butt him with their horns, and (those animals will come in circle): When the last does its turn, the first will start again, and this punishment will go on till Allah has finished the judgments amongst the people."