Srimad-Bhagavatam: Canto 9 - Chapter 1 - Verse 49

Sanskrit:

पुरूरवस एवासीत् त्रयी त्रेतामुखे नृप ।अग्निना प्रजया राजा लोकं गान्धर्वमेयिवान् ॥ ४९ ॥

ITRANS:

purūravasa evāsīttrayī tretā-mukhe nṛpaagninā prajayā rājālokaṁ gāndharvam eyivān

Translation:

O Mahārāja Parīkṣit, at the beginning of Tretā-yuga, King Purūravā inaugurated a karma-kāṇḍa sacrifice. Thus Purūravā, who considered the yajñic fire his son, was able to go to Gandharvaloka as he desired.

Purport:

In Satya-yuga, Lord Nārāyaṇa was worshiped by meditation (kṛte yad dhyāyato viṣṇum). Indeed, everyone always meditated upon Lord Viṣṇu, Nārāyaṇa, and achieved every success by this process of meditation. In the next yuga, Tretā-yuga, the performance of yajña began (tretāyāṁ yajato mukhaiḥ). Therefore this verse says, trayī tretā-mukhe. Ritualistic ceremonies are generally called fruitive activities. Śrīla Viśvanātha Cakravartī Ṭhākura says that in Tretā-yuga, beginning in the Svāyambhuva-manvantara, ritualistic fruitive activities were similarly manifested from Priyavrata, etc.