“Serve without seeking rewards.”
This isn't a verse from the Yoga Vasistha. It is an idea from commentary on the text. For the text's own words, see the verses we've referenced.
About the text: Yoga Vasistha
The Yoga Vasistha is a long Sanskrit philosophical work, traditionally attributed to Valmiki, in which the sage Vasistha teaches Prince Rama through stories and dialogue. Its six books teach liberation through self-effort and insight. Scholars date it anywhere from about the 6th to the 14th century, and it probably grew over time.