Bhagavad Gita Chapter 13, Verse 13

ज्ञेयं यत्तत्प्रवक्ष्यामि यज्ज्ञात्वामृतमश्नुते | अनादिमत्परं ब्रह्म न सत्तन्नासदुच्यते ||

jñeyaṁ yat tat pravakṣyāmi yaj jñātvā amṛtam aśnute anādi mat-paraṁ brahma na sat tan nāsad ucyate

I shall now declare to you that which is to be known, knowing which you will taste immortality. It is the beginningless Supreme Brahman, which is said to be neither existent nor non-existent.

Krishna begins describing the Supreme (Purusha), that which is to be known. 'Jñeyaṁ yat tat pravakṣyāmi'—I shall declare that which is to be known. 'Yaj jñātvā amṛtam aśnute'—knowing which, you will taste immortality. This isn't physical immortality—it's the realization of your eternal nature, the knower that never dies. 'Anādi mat-paraṁ brahma'—the beginningless Supreme Brahman, beyond Me (as the individual). 'Na sat tan nāsad ucyate'—it is said to be neither existent nor non-existent. This means the Supreme is beyond the categories of existence and non-existence that apply to the field. The field exists (it's there, but temporary). The Supreme is beyond existence and non-existence—it's the source of both. When you know this Supreme, you realize your true nature—the knower that is eternal, beyond the field's limitations.