Srimad Bhagvata Mahapurana: Book 1: Chapter 3: Verses 1-14
The Lord's Avataras or Descents into the world of matter Suta says : In the beginning (of creation), with the desire of evolving the different worlds, the Lord assumed the form of Purusa (the Primal Person) consisting of sixteen component principles (viz., the ten sense-organs, the five elements and mind) and constituted of Mahat (the cosmic intellect) etc. While He (the aforesaid Purusa) was displaying His sleep of Samadhi (absorption into or communion with the Self) reposing on the causal waters, there appeared from the lake of His navel a lotus wherefrom sprang up Brahma, the lord of progenitors of the world. It is on the disposition of His limbs that the various worlds stand superimposed; that is the most exalted form of the Lord, consisting of pure Sattva or goodness. The Yogis behold that form by means of their divine eye, wonderful as it is with its thousands of feet, thighs, arms and faces; it has thousands of heads, ears, eyes and noses and is resplendent with thousands of diadems, robes and ear-rings. This form of the Lord (popularly known by the name of Narayana) is the imperishable seed of the various Avataras and the abode to which they all return (after accomplishing their purpose). It is by a ray (e.g., Marici) of His ray (Brahma) that gods, human beings and the lower forms of life are created.
It was this very Lord (Narayana) who, first manifesting Himself as the Kumaras (Sanaka and others), took the form of four Brahmanas and observed the hard vow of perpetual celibacy. During the second Avatara (manifestation) the Lord of all sacrifices assumed the form of the divine Boar with a view to lifting the earth that had sunk into the lowest depths of the ocean, in order to proceed with the work of creation. Appearing in the person of the celestial sage (Narada) in His third manifestation as a Rsi, He taught the Gospel of the Vaisnavas (the Pancaratra), which inculcates the method of doing things in such a way that they may cease to bind us. During His fourth manifestation, in which He was born of Dharma's better half (Murti), He appeared in the dual form of the sages Nara and Narayana and practised severe penance with perfect control of the mind and senses. His fifth manifestation was known by the name of Kapila, the lord of the Siddhas (perfect ones), who taught to the sage Asuri the Sankhya system of philosophy, that determines the nature of the fundamental principles and had been cast into oblivion through the ravages of time. During His sixth manifestation He appeared as a son of the sage Atri at the entreaty of Anasuya (Atri's wife) and taught the Science of the Self to king Alarka, Prahlada and others. Thereafter in the seventh manifestation He was born Akuti, the spouse of Ruci (one of the lords of created beings), as Yajna; assisted by His sons Yama and the other gods He held the office of lndra during the first Manwantara, presided over by Swayambhuva Manu. In the eighth manifestation (as Rsabhadeva) the all-pervading Lord was born of queen Meru Devi, the spouse of king Nabhi, and taught by His own example the mode of life of the Paramahamsas (enlightened souls who have transcended all bounds of morality and have no duty to perform), which is adored by men belonging to all the four Asramas or stages of life. During His ninth descent, at the solicitation of the seers, He took the form of a king (Prthu) and made the earth (in the form of a cow) yield all its products (which she had so far withheld), and hence this particular manifestation of the Lord proved most propitious to the world.