This quote is taken from a 1919 interview with violin great Jascha Heifetz where he touches on a wide range of very important topics including not practicing too much, developing technical mastery on your instrument, and programming while keeping the audience in mind.
There are about 20 quotes I could pull from the interview to highlight but here's another that I really liked:
“Violin Mastery? To me it means the ability to make the violin a perfectly controlled instrument guided by the skill and intelligence of the artist, to compel it to respond in movement to his every wish. The artist must always be superior to his instrument, it must be his servant, one that he can do with what he will."
You can find the entire interview here.