Proverbs 27:17

Iron sharpeneth iron - As hard iron, viz., steel, will bring a knife to a better edge when it is properly whetted against it: so one friend may be the means of exciting another to reflect, dive deeply into, and illustrate a subject, without which whetting or excitement, this had never taken place. Had Horace seen this proverb in the Septuagint translation when he wrote to the Pisos?

Ergo fungar vice cotis, acutum

Reddere quae ferrum valet, exors ipsa secandi