Bhagavad Gita · 15.11

यतन्तो योगिनश्चैनं पश्यन्त्यात्मन्यवस्थितम्।यतन्तोऽप्यकृतात्मानो नैनं पश्यन्त्यचेतसः॥

yatanto yoginaśh chainaṁ paśhyanty ātmany avasthitam yatanto 'py akṛtātmāno nainaṁ paśhyanty achetasaḥ

Yogis who strive can perceive the self dwelling within. But those of unpurified mind, despite their efforts, cannot perceive it.

— Bhagavad Gita 15.11

Source: Sacred-Texts.com (Sanskrit) + Bhagavad-Gita.io (translations)