In Buddhism, there is an analogy of how difficult it is to attain a Human Birth. Or imagine that a small ring is floating on the ocean. On the bottom of the ocean there lives a special turtle that surfaces briefly only once every hundred years. The probability of its head surfacing within the ring is pretty slim, but far greater than the chance of obtaining a precious human body.
The Four Thoughts That Turn The Mind From Samsara by Lopon Tsechu Rinpoche
Let us say that a turtle lives in an ocean for a hundred times a hundred years. Floating upon that ocean is a single yoke with a hole in it, blown by the wind so that it did not stay in one place for even a moment. It is very unlikely that the turtle's throat will be thrust into it. But obtaining a human body from within the lower realms of samsara is taught to be far more difficult. The Spiritual Letter says:
It is harder to gain a human birth and the Dharma,
From the state of having been an animal,
Than for a turtle to put its head into a yoke
While both of them are lost in the vastness of the ocean.
Therefore with these faculties of human beings
By practicing holy Dharma let us reach its fruition.
The Bodhicharyavatara says: 4.20
This is the reason why the Bhagavan has taught
That attaining human birth is much more difficult
Than for a turtle to put its head into a yoke,
Tossed within the vastness of a limitless ocean.
As for the scripture they are speaking about, the Bunch of Flowers says:
It is difficult for the Buddha Bhagavats to enter into the world. But very much more difficult than that is attaining human birth. Let the reason for this be taught in an example. O Shariputra, let the great difficulty of the first be like an ocean. Within it let there be a yoke, having a single hole. Let there also be a decrepit turtle. In that great ocean the wind blows from above and blows from below, and as it blows these things about, that decrepit turtle rises out of the ocean once in a hundred times a hundred years. The difficulty of becoming human again after having fallen back is not equal to that of the throat of that decrepit turtle that rises once in a hundred times a hundred years quickly entering into the hole of that quickly moving yoke. For those who fall away like that, becoming human again is very much more difficult.
The commentary on THE GREAT PERFECTION: THE NATURE OF MIND, THE EASER OF WEARINESS called the Great Chariot
It is taught that a human birth is precious in relation to time, number, and example. “Time” refers to the fact that a human body is very rare. Looking at history and biographies, one learns that a human lifespan is very short. “Number” refers to the number of those few living beings who have acquired a human body in comparison to the inconceivable number of sentient beings who are not able to receive the teachings and practice them. “Example is best illustrated in “The Bodhicharyavatara” by Shantideva, in which he states: “For these reasons, the Buddha has said that for a turtle to insert its neck into a yoke adrift upon the vast ocean, it is just as extremely hard to attain the human state.” In explanation: A blind turtle dwelling on the ocean’s ground has no ambitions; there is a wooden yoke with a hole which tosses about on the great ocean’s surface. The Buddha taught that it is just as difficult to attain a human birth as it is for the turtle to stick its neck through the hole of that yoke when it happens to rise to the surface of the ocean every one hundred years – in fact, it is less likely to attain a precious human body.
His Eminence Jamgon Kongtrul Rinpoche the Third, Karma Lodrö Chökyi Senge. The Supplication - Instructions on “Calling the Lama from Afar by Jamgon Kongtrul Lodrö Thaye the Great
An example is mentioned in Engaging in the Conduct of Bodhisattvas:
For these very reasons, the Buddha has said
That hard as it is for a turtle to insert its neck
Into a yoke adrift upon the vast ocean,
It is extremely hard to attain the human state.
This was explained by the Buddha in the sutras:
Suppose that this whole earth were an ocean and a person threw in a yoke that had only one hole. The yoke would float back and forth in all the four directions. Underneath that ocean, there is a blind tortoise who lives for many thousands of years but who comes up above the surface once every hundred years. It would be very difficult for the tortoise’s head to meet with the yoke’s hole; still, it is possible. To be born in a precious human life is much more difficult.
The Jewel Ornament of Liberation
The Jewel Ornament of Liberation