The Jataka, Vol. III, tr. by H.T. Francis and R.A. Neil, , at sacred-texts.com
"Should a flame sweep," etc.—This story was told by the Master at Jetavana, concerning a slothful Brother. He was, it was said, of gentle birth and lived at Sāvatthi. And after giving a hearty assent to the doctrine and taking orders, he became slothful, and as regards rehearsal of the Law, catechizing, enlightened devotion and the round of priestly duties, he did not fully enter into them, being overcome by his besetting sins, and was always to be found at public lounging-places. The Brethren discussed his sloth in the Hall of Truth, saying, "Such an one, Sirs, after taking orders in so excellent a faith that leads to Salvation, is continually slothful and indolent, and overcome by his besetting sins."  When the Master came and inquired what the Brethren were assembled to discuss, on being told what it was, he said, "Not now only, Brethren, but formerly too was he slothful." And so saying he told an old-world tale.
Once upon a time when Brahmadatta was reigning in Benares, the Bodhisatta became his valued minister. The king of Benares was of a slothful disposition, and the Bodhisatta went about considering some means to rouse the king. Now one day the king went to his garden, accompanied by his minister, and while wandering about there he espied a slothful tortoise. Lazy creatures like these, they say, though they are in motion a whole day, move only just an inch or two.
The king on seeing it asked, saying, "Friend, what is its name?"
The Bodhisatta answered, "The creature is called a tortoise, great king; and is so lazy that though it is in motion all day, it only moves just an inch or two." And addressing it he said, "Ho! Sir Tortoise, yours is a slow motion. Supposing a conflagration arose in the forest, what would you do?" And herewith he spoke the first stanza:
Leaving blackened path behind,
How, Sir Waddler, slow to move,
Way of safety couldst thou find?
The tortoise on hearing this repeated the second stanza:
Chinks there be in every tree,
Here a refuge will be found
Or an end of us ’twill be.
 On hearing this the Bodhisatta gave utterance to two stanzas:
And tarries long when utmost speed is best,
Destroys the slender fabric of his weal,
As withered leaf is crushed beneath the heel.
But they who wait betimes nor haste too soon,
Fulfil their purpose, as her orb the moon.
The king, hearing the words of the Bodhisatta, thenceforth was no longer indolent.
The Master, having ended his lesson, identified the Birth: "At that time the slothful Brother was the tortoise, and I myself was the wise councillor."