In this excerpt from the Panchavimsha Brahmana of the Sama Veda, aka the Tandya Mahabrahmana, the performance of a 100-year Yagna is described. It says that this Yagna was once performed by the Sadhyas, a group of gods born in the Chakshusha Manvantara and which may include an incarnation of Vishnu as I discuss here and here. In any case, the Panchavimsha Brahmana says the Sadhyas used this 100-year Yagna to go to Devaloka:
Before the (time of the present) Gods, there were (certain) Gods called Sadhyas. They undertook this sacrificial session; through this they throve. Together with their cattle and their people, they all went together to the world of heaven. In the same manner, forsooth, those who under take this (sattra) go all together to the world of heaven. This sacrificial session is (equal in duration to) the normal life (of man). Man reaches a hundred years. As long as is life that they (the performers of this rite) reach. For no sacrificial session is longer than (the duration of) life. With regard to this, a (verse) occurs (in the sacred texts): "Those days were numerous that in the east (have risen) at sunrise whence Dawn, approaching, as it were, her lover (the sun), has shown herself as not returning."
The quote at the end is from verse 3 of Rig Veda Book 7 Hymn 76:
tānīdahāni bahulānyāsan yā prācīnamuditā sūryasya |
yataḥ pari jāra ivācarantyuṣo dadṛkṣe na punaryatīva ||
Great is, in truth, the number of the Mornings which were aforetime at the Sun's uprising.
Since thou, O Dawn, hast been beheld repairing as to thy love, as one no more to leave him.
As you can see in the Rig Veda Anukramani in my answer here, this hymn was heard by the sage Vasishta and it's addressed to Usha, goddess of the dawn.
But my question is, why does the Panchavimsha Brahmana quote this Rig Veda verse in regard to the 100-year Yagna of the Sadhyas? I don't see any connection between this verse and the story of the Sadhyas. Is it possible that the verse is saying that he who performs the 100-year Yagna will live to see a great many sunrises?
Unfortunately Sayana's commentary on this Rig Veda verse just says this:
Like a wife: yatha pari jaraivacaranti... na punar yativa: as in the world a virtuous woman is not seen abandoning a bad and vagrant husband, nor herself going astray.
In any case, does anyone know of any commentaries on the Panchavimsha Brahmanas which shed light on this?