One good turn deserves another
The intrigue here is not the meaning that kindness should be repaid with kindness but rather the origin. It is fist cited as an old English proverb from at least the 1400s but some sources maintain it existed long before this in Latin. The original Latin is manus manum lavat, literally, one hand washes another, which is not quite the same meaning and this Latin phrase is attributed to two authors: Gaius Petronius Arbiter in his work The Satyricon, but also to his contemporary Seneca. Both these Latin citations date from the first century AD, but the variation in meaning makes the old English proverb from c.1400 the more favoured origin.