The only document assigned to Pelagius that I could find is his letter to Demetrias, a fourteen year old girl of wealthy family who shunned riches and made enquiries regarding her own religious desires.
The letter is presented online but the two presentations I have found (herein linked) are very different and there is some question about the accuracy of translation from the original Latin.
In one presentation I found the words :
But the joy of heaven, which is the reward for making good and wise choices on earth, lasts for all eternity. And the agony of hell, which is the punishment for making bad choices on earth, also lasts for all eternity. The day of judgement, when God will decide where to send us, is imminent; even if a person is young, and destined to live for a hundred years, the day is still imminent.
But the tone of the second presentation is different, the document is much longer and I am unable to find the above quoted words within it, so there is doubt about the accuracy of the documentation available.
The second link provides a copy of the original Latin written by Pelagius.
Grateful thanks to @Joseph Hinkle for providing the second link.