How to recognize a true believer, from Matthew 7:16-20:

16 By their fruit you will recognize them. Do people pick grapes from thornbushes, or figs from thistles? 17 Likewise, every good tree bears good fruit, but a bad tree bears bad fruit. 18 A good tree cannot bear bad fruit, and a bad tree cannot bear good fruit. 19 Every tree that does not bear good fruit is cut down and thrown into the fire. 20 Thus, by their fruit you will recognize them.

I mean, I can only assume that what they are referring to is Bhagavad Gita 9:27:

Whatever you do, whatever you eat, whatever you offer in sacrifice, whatever you give, whatever austerity you practice, O Arjuna, do it as an offering to Me [Lord Krishna].