Srimad Bhagvata Mahapurana: Book 12: Chapter 2: Verses 12-24
When through the evil effect of Kali the bodies of men get reduced in size and emaciated, the righteous course chalked out by the Vedas for men following the system of Varnas (grades of society) and Asramas (stages in life) gets lost, when religion is replaced by heresy to a large extent and rulers mostly turn out to be thieves, when men take to various pursuits like theft, mendacity, wanton destruction of life and so on; when members of the three (higher) castes are mostly converted into Sudras and cows are reduced to the size of she-goats and begin to yield as much milk; when people belonging to the four Asramas mostly become householders and the conotation of relationship will extend only to the relatives of one's wife; annual plants get stunted in growth and trees are mostly reduced to the size of a Sarni (a small tree); clouds will mostly end in flashes of lightning (rather than pour rain) and dwellings will mostly look desolate (for want of hospitality to strangers); in this way when the Kali age, whose career is so severe to the people, is well-nigh past, the Lord will appear in His divine form (consisting of Sattva alone) for the protection of virtue. Lord Visnu, adored of the whole animate and inanimate creation, and the Soul of the universe,, appears (in this world of matter) for protecting the virtue of the righteous and wiping out (the entire stock of) their Karma (and thereby liberating them). The Lord will appear under the name of Kalki in the house of the high-souled Visnuyasa-the foremost Brahmana of the village of Sambhala. Riding a fleet horse named Devadatta (obviously because it will be presented to Him by the gods) and capable of subduing the wicked, the Lord of the universe, wielding (the well-known) eight divine powers (Anima and so on) and possessed of (endless) virtues and matchless splendour, will traverse the globe on that swift horse and exterminate with His sword in tens of millions robbers wearing the insignia of royalty. Now when all the robbers are (thus) exterminated, the minds of the people of the cities and the countryside will become pure indeed because of their enjoying the breezes wafting the most sacred fragrance of pigments on the person of Lord Vasudeva. With Lord Vasudeva, the embodiment of strength, in their heart their progeny will grow exceedingly strong (as before). When Lord Sri Hari, the Protector of Dharma, appears as Kalkir Satyayuga will prevail (once more) at that time and the progeny of the people will be of a Sattvika (virtuous) disposition. When the moon, the sun and the Jupiter rise together as) Satyayuga.