Mbh 1.100.8a

“Vaisampayana continued, ‘O Bharata, when Santanu heard this, he felt no inclination to grant such a boon, though the fire of desire sorely burnt him within. The king with his heart afflicted by desire returned to Hastinapura, thinking all the way of the fisherman’s daughter. And having returned home, the monarch passed his time in sorrowful meditation. One day, Devavrata approaching his afflicted father said, ‘All is prosperity with thee; all chiefs obey thee; then how is it that thou grievest thus? Absorbed in thy own thoughts, thou speakest not a word to me in reply. Thou goest not out on horse-back now; thou lookest pale and emaciated, having lost all animation. I wish to know the disease thou sufferest from, so that I may endeavour to apply a remedy.’ Thus addressed by his son, Santanu answered, ‘Thou sayest truly, O son, that I have become melancholy. I will also tell thee why I am so. O thou of Bharata’s line, thou art the only scion of this our large race. Thou art always engaged in sports of arms and achievements of prowess. But, O son, I am always thinking of the instability of human life. If any danger overtake thee, O child of Ganga, the result is that we become sonless. Truly thou alone art to me as a century of sons. I do not, therefore, desire to wed again. I only desire and pray that prosperity may ever attend thee so that our dynasty may be perpetuated. The wise say that he that hath one son hath no son. Sacrifices before fire and the knowledge of the three Vedas yield, it is true, everlasting religious merit, but all these, in point of religious merit, do not, come up to a sixteenth part of the religious merit attainable on the birth of a son. Indeed, in this respect, there is hardly any difference between men and the lower animals. O wise one, I do not entertain a shadow of doubt that one attains to heaven in consequence of his having begotten a son.