Srimad Bhagavatam Canto 09, Chapter 14, Text 46

SB 9.14.46

tasya nirmanthanaj jato
 jata-veda vibhavasuh
trayya sa vidyaya rajña
 putratve kalpitas tri-vrt
 
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
From Pururava’s rubbing of the aranis came a fire. By such a fire one can achieve all success in material enjoyment and be purified in seminal birth, initiation and in the performance of sacrifice, which are invoked with the combined letters a-u-m. Thus the fire was considered the son of King Pururava.
 
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada: 
 
According to the Vedic process, one can get a son through semen (sukra), one can get a bona fide disciple through initiation (savitra), or one can get a son or disciple through the fire of sacrifice (yajña). Thus when Maharaja Pururava generated the fire by rubbing the aranis, the fire became his son. Either by semen, by initiation or by yajña one may get a son. The Vedic mantra omkara, or pranava, consisting of the letters a-u-m, can call each of these three methods into existence. Therefore the words nirmanthanaj jatah indicate that by the rubbing of the aranis a son was born.
Srimad Bhagavatam Canto 09, Chapter 14, Text 44-45
Srimad Bhagavatam Canto 09, Chapter 14, Text 47