§36. "And what is the faculty of persistence? There is the case where a monk, a disciple of the noble ones, keeps his persistence aroused for abandoning unskillful mental qualities and taking on skillful mental qualities. He is steadfast, solid in his effort, not shirking his duties with regard to skillful mental qualities. He generates desire, endeavors, arouses persistence, upholds & exerts his intent for the sake of the non-arising of evil, unskillful qualities that have not yet arisen... for the sake of the abandoning of evil, unskillful qualities that have arisen... for the sake of the arising of skillful qualities that have not yet arisen... (and) for the maintenance, non-confusion, increase, plenitude, development, & culmination of skillful qualities that have arisen. This is called the faculty of persistence."
— SN 48.10
§37. "And how is striving fruitful, how is exertion fruitful? There is the case where a monk, when not loaded down, does not load himself down with pain, nor does he reject pleasure that accords with the Dhamma, although he is not infatuated with that pleasure. He discerns that "When I exert a [physical, verbal, or mental] fabrication against this cause of stress, then from the fabrication of exertion there is dispassion (fading away). When I look on with equanimity at that cause of stress, then from the development of equanimity there is dispassion." So he exerts a mental fabrication against the [first] cause of stress... and develops equanimity with regard to the [second] cause of stress... Thus the stress [coming from any cause of the first sort] is abolished... & the stress [coming from any cause of the second sort] is abolished."
— MN 101
§38. On that occasion Ven. Sona was staying near Rajagaha in the Cool Wood. Then, as Ven. Sona was meditating in seclusion [after doing walking meditation until the skin of his soles was split & bleeding], this train of thought arose in his awareness: "Of the Blessed One's disciples who have aroused their persistence, I am one, but my mind is not released from fermentations through lack of clinging/sustenance. Now, my family has enough wealth that it would be possible to enjoy wealth & make merit. What if I were to disavow the training, return to the lower life, enjoy wealth, & make merit?"
Then the Blessed One, as soon as he perceived with his awareness the train of thought in Ven. Sona's awareness disappeared from Vulture Peak Mountain — just as a strong man might extend his flexed arm or flex his extended arm — appeared in the Cool Wood right in front of Ven. Sona, and sat down on a prepared seat. Ven. Sona, after bowing down to the Blessed One, sat to one side. As he was sitting there, the Blessed One said to him, "Just now, as you were meditating in seclusion, didn't this train of thought appear to your awareness: 'Of the Blessed One's disciples who have aroused their persistence, I am one, but my mind is not released from fermentations... What if I were to disavow the training, return to the lower life, enjoy wealth, & make merit?'"
"Yes, lord."
"Now what do you think, Sona. Before, when you were a house-dweller, were you skilled at playing the vina?"
"Yes, lord."
"And what do you think: when the strings of your vina were too taut, was your vina in tune & playable?"
"No, lord."
"And what do you think: when the strings of your vina were too loose, was your vina in tune & playable?"
"No, lord."
"And what do you think: when the strings of your vina were neither too taut nor too loose, but tuned [lit: 'established'] to be right on pitch, was your vina in tune & playable?"
"Yes, lord."
"In the same way, Sona, over-aroused persistence leads to restlessness, overly slack persistence leads to laziness. Thus you should determine the right pitch for your persistence, attune the pitch of the (five) faculties (to that), and there pick up your theme."
"Yes, lord," Ven. Sona answered the Blessed One. Then, having given this exhortation to Ven. Sona, the Blessed One — as a strong man might extend his flexed arm or flex his extended arm — disappeared from the Cool Wood and appeared on Vulture Peak Mountain.
So after that, Ven. Sona determined the right pitch for his persistence, attuned the pitch of the [five] faculties [to that], and there picked up his theme. Dwelling alone, secluded, heedful, ardent, & resolute, he in no long time reached & remained in the supreme goal of the holy life for which clansmen rightly go forth from home into homelessness, knowing & realizing it for himself in the here & now. He knew: "Birth is ended, the holy life fulfilled, the task done. There is nothing further for the sake of this world." And thus Ven. Sona became another one of the arahants.
— AN 6.55
§39.
Ven. Matangaputta
It's too cold, too hot, too late in the evening — people who say this, shirking their work: the moment passes them by. Whoever regards cold & heat as no more than grass, doing his manly duties, won't fall away from ease. With my chest I push through wild grasses — spear-grass, ribbon-grass, rushes — cultivating a heart bent on seclusion.
— Thag 3.5