Srimad Bhagavatam Canto 04, Chapter 23, Text 04

SB 4.23.4

tatrapy adabhya-niyamo
arabdha ugra-tapasi
yatha sva-vijaye pura
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
After retiring from family life, Maharaja Prthu strictly followed the regulations of retired life and underwent severe austerities in the forest. He engaged in these activities as seriously as he had formerly engaged in leading the government and conquering everyone.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
As it is necessary for one to become very active in family life, similarly, after retirement from family life, it is necessary to control the mind and senses. This is possible when one engages himself fully in the devotional service of the Lord. Actually the whole purpose of the Vedic system, the Vedic social order, is to enable one to ultimately return home, back to Godhead. The grhastha-asrama is a sort of concession combining sense gratification with a regulative life. It is to enable one to easily retire in the middle of life and engage fully in austerities in order to transcend material sense gratification once and for all. Therefore in the vanaprastha stage of life, tapasya, or austerity, is strongly recommended. Maharaja Prthu followed exactly all the rules of vanaprastha life, which is technically known as vaikhanasa-asrama. The word vaikhanasa-susammate is significant because in vanaprastha life the regulative principles are also to be strictly followed. In other words, Maharaja Prthu was an ideal character in every sphere of life. Mahajano yena gatah sa panthah: one should follow in the footsteps of great personalities. Thus by following the exemplary character of Maharaja Prthu, one can become perfect in all respects while living this life or while retiring from active life. Thus after giving up this body, one can become liberated and go back to Godhead.
Srimad Bhagavatam Canto 04, Chapter 23, Text 01-03
Srimad Bhagavatam Canto 04, Chapter 23, Text 05