[15] Furthermore, bhikkhus, a bhikkhu reflects thus: 'While I dwell as I please, unwholesome qualities (akusalā dhammā) increase and wholesome qualities (kusalā dhammā) decline; but while I exert myself in what is painful, unwholesome qualities decline and wholesome qualities increase. Suppose, then, I were to exert myself in what is painful.' He exerts himself in what is painful. While he is exerting himself in what is painful, unwholesome qualities decline and wholesome qualities increase. At a later time he does not exert himself in what is painful. Why is that? Because, bhikkhus, the purpose for the sake of which that bhikkhu would exert himself in what is painful has been achieved; therefore at a later time he does not exert himself in what is painful. Just as, bhikkhus, an arrowsmith heats and warms an arrow shaft between two firebrands, making it straight and workable; and when, bhikkhus, the arrow shaft has been heated and warmed by the arrowsmith between two firebrands, made straight and workable, at a later time that arrowsmith does not heat and warm the arrow shaft between two firebrands, making it straight and workable — why is that? Because, bhikkhus, the purpose for the sake of which the arrowsmith would heat and warm the arrow shaft between two firebrands, making it straight and workable, has been achieved; therefore at a later time the arrowsmith does not heat and warm the arrow shaft between two firebrands, making it straight and workable — even so, bhikkhus, a bhikkhu reflects thus: 'While I dwell as I please, unwholesome qualities increase and wholesome qualities decline; but while I exert myself in what is painful, unwholesome qualities decline and wholesome qualities increase. Suppose, then, I were to exert myself in what is painful.' He exerts himself in what is painful. While he is exerting himself in what is painful, unwholesome qualities decline and wholesome qualities increase. At a later time he does not exert himself in what is painful. Why is that? Because, bhikkhus, the purpose for the sake of which that bhikkhu would exert himself in what is painful has been achieved; therefore at a later time he does not exert himself in what is painful. In this way too, bhikkhus, exertion is fruitful, striving is fruitful.
English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.