Bhagavad Gita · 18.38

विषयेन्द्रियसंयोगाद्यत्तदग्रेऽमृतोपमम्।परिणामे विषमिव तत्सुखं राजसं स्मृतम्॥

viṣhayendriya-saṁyogād yat tad agre 'mṛitopamam pariṇāme viṣham iva tat sukhaṁ rājasaṁ smṛitam

That happiness which arises from the contact of the senses with their objects, which is like nectar in the beginning but like poison in the end — that happiness is declared to be rajasic.

— Bhagavad Gita 18.38

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