The Lute
Aṅguttara Nikāya 130 (VI, 55)
A monk called Sona is about to quit. The Buddha appears before him and asks him “Did you used to play the lute?” to which the answer was “yes”.
“Was it easy to play when the strings were too taut?” Sona says “no”.
“Was it easy to play when the strings were too loose?” Sona says “no”.
“Was it easy to play when the strings were just right?” Sona says “yes”.
“You should balance your energy in practice like tuning a lute with the tension neither too taut nor too loose.”
Sona later becomes one of the arahants.