Bhagavad Gita Chapter 18, Verse 37

यत्तदग्रे विषमिव परिणामेऽमृतोपमम् | तत्सुखं सात्त्विकं प्रोक्तमात्मबुद्धिप्रसादजम् ||

yat tad agre viṣam iva pariṇāme 'mṛtopamam tat sukhaṁ sāttvikaṁ proktam ātmabuddhi-prasādajam

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

Krishna explains sattvic happiness. 'Yat tad agre viṣam iva pariṇāme 'mṛtopamam'—that happiness which is like poison (viṣam iva) in the beginning (agre) but like nectar (amṛta-upamam) in the end (pariṇāme). 'Tat sukhaṁ sāttvikaṁ proktam ātmabuddhi-prasādajam'—that happiness (tat sukham) is declared (proktam) to be sattvic (sāttvikam), born of clarity of self-knowledge (ātma-buddhi-prasādajam). This is sattvic happiness: initially difficult, like poison, but ultimately like nectar, born of clarity of self-knowledge. This is the highest kind of happiness: it requires effort and discipline initially, but leads to lasting joy and the end of suffering. Unlike rajasic happiness (which is pleasurable but temporary) and tamasic happiness (which is deluded), sattvic happiness is born of understanding the self and leads to true, lasting happiness. This is the path: initially difficult practice that leads to nectar-like joy.