In their former birth, these two sons, known as Nalakūvara and Maṇigrīva, were extremely opulent and fortunate. But because of pride and false prestige, they did not care about anyone, and thus Nārada Muni cursed them to become trees.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
पुरा नारदशापेन वृक्षतां प्रापितौ मदात् । नलकूबरमणिग्रीवाविति ख्यातौ श्रियान्वितौ
purā nāradaśāpena vṛkṣatāṃ prāpitau madāt | nalakūbaramaṇigrīvāviti khyātau śriyānvitau
SB 10.9.23