O Viṣṇudatta, Parīkṣit Mahārāja, the Supreme Personality of Godhead was pleased by the great sages at that sacrifice. Consequently the Lord decided to personally exhibit the method of executing religious principles [as observed by brahmacārīs, sannyāsīs, vānaprasthas and gṛhasthas engaged in rituals] and also satisfy Mahārāja Nābhi’s desire. Consequently He appeared as the son of Merudevī in His original spiritual form, which is above the modes of material nature.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
बर्हिषि तस्मिन्नेव विष्णुदत्त भगवान् परमर्षिभिः प्रसादितो नाभेः प्रियचिकीर्षया तदवरोधायने मेरुदेव्यां धर्मान् दर्शयितुकामो वातरशनानां श्रमणानामृषीणामूर्ध्वमन्थिनां शुक्लया तनुवावततार
barhiṣi tasminneva viṣṇudatta bhagavān paramarṣibhiḥ prasādito nābheḥ priyacikīrṣayā tadavarodhāyane merudevyāṃ dharmān darśayitukāmo vātaraśanānāṃ śramaṇānāmṛṣīṇāmūrdhvamanthināṃ śuklayā tanuvāvatatāra
SB 5.3.20