SB 9.14.43
sthalim nyasya vane gatva
grhan adhyayato nisi
tretayam sampravrttayam
manasi trayy avartata
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
King Pururava then left Agnisthali in the forest and returned home, where he meditated all night upon Urvasi. In the course of his meditation, the Treta millennium began, and therefore the principles of the three Vedas, including the process of performing yajña to fulfill fruitive activities, appeared within his heart.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
It is said, tretayam yajato makhaih: in Treta-yuga, if one performed yajñas, he would get the results of those yajñas. By performing visnu-yajña specifically, one could even achieve the lotus feet of the Supreme Personality of Godhead. Of course, yajña is intended to please the Supreme Personality of Godhead. While Pururava was meditating upon Urvasi, the Treta-yuga began, and therefore the Vedic yajñas were revealed in his heart. But Pururava was a materialistic man, especially interested in enjoying the senses. Yajñas for enjoyment of the senses are called karma-kandiya-yajñas. Therefore, he decided to perform karma-kandiya-yajñas to fulfill his lusty desires. In other words, karma-kandiya-yajñas are meant for sensuous persons, whereas yajña should actually be performed to please the Supreme Personality of Godhead. To please the Supreme Personality of Godhead in Kali-yuga, the sankirtana-yajña is recommended. Yajñaih sankirtana-prayair yajanti hi sumedhasah. Only those who are very intelligent take to sankirtana-yajña to fulfill all their desires, material and spiritual, whereas those who are lusty for sense enjoyment perform karma-kandiya-yajñas.