'ता एवापो ददौ तस्य स ऋषिः शंसितव्रतः' इति महाकौर्मे
This is quoted in Sri Madhwacharya's Geetha Bhashya (2-50)
The story is like this:
Lord Indra handovers his Kamandalu of water to Vasishtha rishi.
The rishi giving back the same water from his Kamandalu to Indra.
If someone can narrate this story clearly, I will be grateful to them.