SB 1.3.9

turye dharma-kalā-sarge
 nara-nārāyaṇāv ṛṣī
bhūtvātmopaśamopetam
 akarot duścaraṁ tapaḥ
Synonyms: 
turye — in the fourth of the line; dharma-kalā — wife of Dharmarāja; sarge — being born of; nara-nārāyaṇau — named Nara and Nārāyaṇa; ṛṣī — sages; bhūtvā — becoming; ātma-upaśama — controlling the senses; upetam — for achievement of; akarot — undertook; duścaram — very strenuous; tapaḥ — penance.
Translation: 
In the fourth incarnation, the Lord became Nara and Nārāyaṇa, the twin sons of the wife of King Dharma. Thus He undertook severe and exemplary penances to control the senses.
Purport: 

As King Ṛṣabha advised His sons, tapasya, or voluntary acceptance of penance for realization of the Transcendence, is the only duty of the human being; it was so done by the Lord Himself in an exemplary manner to teach us. The Lord is very kind to the forgetful souls. He therefore comes Himself and leaves behind necessary instructions and also sends His good sons as representatives to call all the conditioned souls back to Godhead. Recently, within the memory of everyone, Lord Caitanya also appeared for the same purpose: to show special favor to fallen souls of this age of iron industry. The incarnation of Nārāyaṇa is worshiped still at Badarī-nārāyaṇa, on the range of the Himālayas.