The Jataka, Volume I, tr. by Robert Chalmers, , at sacred-texts.com
"They twain in fisher's net."--This story was told by the Master while at Jetavana, about two aged Elders. After a rainy-season spent in a forest in the country they resolved to seek out the Master, and got together provisions for their journey. But they kept putting off their departure day by day, till a month flew by. Then they provided a fresh supply of provisions, and procrastinated till a second month was gone, and a third. When their indolence and sluggishness had lost them three months, they set out and came to Jetavana. Laying aside their bowls and robes in the common-room, they came into the Master's presence. The Brethren remarked on the length of the time since the two had visited the Master, and asked the reason. Then  they told their story and all the Brotherhood came to know of the laziness of these indolent Brethren.
Assembling in the Hall of Truth the Brethren talked together of this thing. And the Master entered and was told what they were discussing. Being asked whether they were really so indolent, those Brethren admitted their short-coming. "Brethren," said he, "in former times, no less than now, they were indolent and loth to leave their abode." So saying, he told this story of the past.
Once on a time when Brahmadatta was reigning in Benares, there lived in the river of Benares three fishes, named Over-thoughtful, Thoughtful, and
[paragraph continues] Thoughtless. And they came down-stream from the wild country to where men dwelt. Hereupon Thoughtful said to the other two, "This is a dangerous and perilous neighbourhood, where fishermen catch fish with nets, basket-traps, and such like tackle. Let us be off to the wild country again." But so lazy were the other two fishes, and so greedy, that they kept putting off their going from day to day, until they had let three months slip by. Now fishermen cast their nets into the river; and Over-thoughtful and Thoughtless were swimming on ahead in quest of food when in their fully they blindly rushed into the net. Thoughtful, who was behind, observed the net, and saw the fate of the other two.
"I must save these lazy fools from death," thought he. So first he dodged round the net, and splashed in the water in front of it like a fish that has broken through and gone up stream; and then doubling back, he splashed about behind it, like a fish that has broken through and gone down stream. Seeing this, the fishermen thought the fish had broken the net and all got away; so they pulled it in by one corner and the two fishes escaped from the net into the open water again. In this way they owed their lives to Thoughtful.
His story told, the Master, as Buddha, recited this stanza:
His lesson ended, and the Four Truths expounded (at the close whereof the aged Brethren gained fruition of the First Path), the Master identified the Birth by saying: "These two Brethren were then Over-thoughtful and Thoughtless, and I Thoughtful."