When the primeval Lord Nārāyaṇa created His universal body out of the five elements produced from Himself and then entered within that universal body by His own plenary portion, He thus became known as the Puruṣa.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
भूतैर्यदा पञ्चभिरात्मसृष्टैः पुरं विराजं विरचय्य तस्मिन् । स्वांशेन विष्टः पुरुषाभिधान- मवाप नारायण आदिदेवः
bhūtairyadā pañcabhirātmasṛṣṭaiḥ puraṃ virājaṃ viracayya tasmin | svāṃśena viṣṭaḥ puruṣābhidhāna- mavāpa nārāyaṇa ādidevaḥ
SB 11.4.3