Māndukya 1.02

सर्वं ह्येतद् ब्रह्म। अयमात्मा ब्रह्म। सोऽयमात्मा चतुष्पात् ॥२॥

All this is indeed Brahman. "This Ātmān is Brahman". This self has four aspects.

Bhāshya 1.02

"All this is indeed Brahman" - indicates that fullness is that of Brahman, and no one else. To explain "This Ātmān is Brahman", a testimonial from Harivamsa is referenced where the utterances of Varuna in the form of a frog specifically explains Māndukyōpanishad. The text suggests that within all bodies, starting from the divine Brahman, there exists an entity perceived as the doer of actions. Observing the lack of independence among beings, it is demonstrated that this entity is indeed the Ātmān, which is Brahman.

"परमं यो महद्ब्रह्म", "तदेव ब्रह्म परमं कवीनाम्", "पूर्णमदः पूर्णमिदम्"

"The supreme entity, known as the great Brahman", "That is indeed the supreme Brahman revered by the sages", "That is whole, this is whole"

इत्यादिषु प्रसिद्धं च ब्रह्मणः पूर्णत्वमित्याह – सर्वं ह्येतत् ब्रह्मेति॥

- in such statements, it is well-known that fullness is of Brahman; thus it is said: "All this is indeed Brahman."

श्रीब्रह्मादिसकलदेहेषु स्थित्वाऽऽदानादिकर्ता योऽयं कश्चित् प्रतीयते। जीवानां अस्वातन्त्र्यदर्शनात् सोऽपि स एवेति दर्शयति – अयमात्मा ब्रह्मेति।

The text suggests that within all bodies, starting from the divine Brahman, there exists an entity perceived as the doer of actions. Observing the lack of independence among beings, it is demonstrated that this entity is indeed the Ātmān, which is Brahman.

"पूर्णस्तु हरिरेवैको नान्यत् पूर्णं कदाचन। विना च प्रकृतिं नान्यत् कालातीतं परात्मनः॥

"Hari alone is complete; nothing else is ever complete. Without Prakrti, nothing else transcends time of the Supreme Ātmān.

कालश्चैव दिशो वेदाः प्रकृत्यात्मान ईरिताः। अभिमानात्तु जीवानां न कालातीतता भवेत्॥

Time, directions, and the Vedas are governed by the combination of Prakṛti and Ātmān. However, due to pride, living beings cannot transcend time.

मुक्तानामपि पूर्वत्र कालसम्बन्ध ईरितः। पूर्णत्वं च सदा विष्णोः प्रसिद्धं सर्ववेदतः॥

Even for the liberated souls, the relationship with time is stated. The completeness and eternal nature of Lord Vishnu are well-known from all the Vedas.

सोऽयं विष्णू रमाब्रह्मरुद्रानन्तादिगः सदा। आदानादनकर्तृत्वादात्मा तेषामगोचरः॥ इति मण्डूकरूपी सन् ददर्श वरुणः श्रुतिम्॥"

Thus, Lord Vishnu, who is always with goddess Lakshmi, Brahma, Rudra, Ananta, and others in the form of Ātmān, is beyond perception due to his giving and taking (as per their tendencies) and he himself being non-partisan in doership. In the form of a frog, Varuna discovered this Vedic knowledge."

इति हरिवंशेषु ॥२॥

- stated thus in the Harivamsa.