The Rahasyatraya Sara is a work by the Sri Vaishnava Acharya Vedanta Desikan, concerning the meaning of the Rahasya Traya or three secrets of the Sri Vaishnava sect (of which I'm a member). In this excerpt from the Rahasyatraya Sara, Vedanta Desikan discusses why Hindu scripture describes other gods as aids to attaining Moksha, when Vishnu is described as the sole path to Moksha:
If, in some places, these gods are spoken of as assisting in the attainment of moksha, it should be understood to mean that they, like Acharyas, assist by imparting knowledge and in other such ways. This is also the purport of the following passage; "The man who is a devotee of the god of the sun (Surya) will, after seven more births, become a devotee of Rudra by the Sun's grace. He who is a devotee of Sankara will, after seven more births, become a devotee of Vishnu by the grace of Sankara." And again:- "He who is a devotee of Vasudeva will, after those seven births, become one with Vasudeva by His grace."
My question is, what scripture contains these quotes about a Saura (devotee of Surya) becoming a Shaivite after seven births, a Shaivite becoming a Vaishnava after seven more births, and a Vaishnava attaining Moksha after yet another seven births?
I expect it's from one of the Puranas, but I'm not sure which one. Are there any commentaries on Vedanta Desikan's work which address this?