Final answer:
The complete verse from the New Testament is Luke 6:35, which exhorts individuals to love their enemies and do good to them without expecting any return, reflecting a reciprocal ethical principle found in various historical teachings.
Step-by-step explanation:
The verse you're referring to is from the Bible, specifically the New Testament. The complete verse is: "But love your enemies, do good to them, and lend to them without expecting to get anything back. Then your reward will be great, and you will be children of the Most High, because he is kind to the ungrateful and wicked." This is found in Luke 6:35. This directive mirrors the sentiments found in various religious and ethical teachings dating back to the 6th century BCE and later, emphasizing the importance of love and kindness towards all individuals, regardless of their relationship to us.