From the Vinaya Pitaka (1, 181-182) via Edward Conze’s Buddhist Texts Through the Ages (later reprinted in Penguin Classics as Buddhist Scriptures)
Sona Kolivisa, a merchant’s son, received his going forth in the Lord’s presence, he received ordination. Because of his great output of vigour while pacing up and down, his feet split and the place for pacing up and down in became stained with blood as though cattle had been slaughtered there.
As the venerable Sona was meditating in private he thought: “The Lord’s disciples, of whom I am one, dwell putting forth vigour; but even so my mind is not freed from the outflows with no (further) clinging, and moreover there are my family’s possessions. Suppose I were to return to the low life, enjoy the possessions and do good?”
The Lord knew by mind the thoughts in the venerable Sona’s mind. He approached him and said: “Sona, formerly when you were a householder were you clever at the lute’s stringed music? ”
“Yes, Lord.”
“When the strings of the lute were too taut, was it tuneful and fit for playing? ”
“Certainly not, Lord.”
“And when they were too slack, was the lute tuneful and fit for playing? ”
“No, Lord.”
“But when the strings were neither too taut nor too slack but were keyed to an even pitch, was your lute tuneful and fit for playing? ”
“Yes, Lord.”
“Even so, Sona, does too much output of vigour conduce to restlessness and too feeble a vigour to slothfulness. Therefore, Sona, determine on evenness in vigour.”
Extracts 