Mbh 3.83.1c

There, O king, the heroic Rama of resplendent energy, exterminating the Kshatriyas by his might, dug five lakes and filled them, O tiger among men, with the blood of his victims, as heard by us. And having filled those lakes with Kshatriya blood, Rama offered oblations of blood to his sires and grandsires. Gratified (with the oblations) those Rishis then addressed Rama and said, ‘O Rama, O Rama, O thou of great good fortune, we have been gratified with thee, O thou of the Bhrigu race, for this thy regard for the Pitris, and thy prowess, O exalted one! Blessed be thou and ask thou the boon thou choosest. What is that thou desirest, O thou of great splendour!’ Thus addressed (by them), Rama, that foremost of smiters, said with joined hands these words unto the Pitris, stationed in the firmament, ‘If ye have been gratified with me, if I have deserved your favour, I desire this favour of the Pitris, viz., that I may have pleasure again in ascetic austerities. Let me also, through your power, be freed from the sin I have committed by exterminating, from wrath, the Kshatriya race. Let also my lakes become tirthas celebrated over the world. The Pitris, hearing these blessed words of Rama, were highly gratified, and filled with joy they answered him saying, ‘Let thy asceticism increase in consequence of thy regard for the Pitris. Thou hast exterminated the Kshatriyas from wrath. Freed art thou already from that sin, for they have perished as a consequence of their own misdeeds. Without doubt, these lakes of thine will become tirthas. And if one, bathing in these lakes, offereth oblations of the water thereof to the Pitris, the latter gratified with him will grant him desire, difficult of fulfilment in the world as also eternal heaven.’ O king, having granted him these boons, the Pitris joyfully saluted Rama of the Bhrigu race and disappeared there and then. It was thus that the lakes of the illustrious Rama of the Bhrigu race became sacred.