Bhagavad Gita · 18.37

यत्तदग्रे विषमिव परिणामेऽमृतोपमम्।तत्सुखं सात्त्विकं प्रोक्तमात्मबुद्धिप्रसादजम्॥

yat tad agre viṣham iva pariṇāme 'mṛitopamam tat sukhaṁ sāttvikaṁ proktam ātma-buddhi-prasāda-jam

That happiness which is like poison in the beginning but like nectar in the end, born of the clarity of self-knowledge — that happiness is declared to be sattvic.

— Bhagavad Gita 18.37

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