Are children required to carry out the deceased's oral will, which they did not hear directly from him, and which asks them to reclaim land that has already been sold, based on the testimony of a single person whom they do not trust?

The desire of the testator that the heirs not sell the land is not a valid, legally enforceable will, because a will is a donation of something that becomes owned by the legatee after the death of the testator, and it is an endowment of ownership that takes effect after death by way of donation. The wording mentioned in the question – that he told the lawyer he would write a will prohibiting the sale of the land – is merely an expression of intent and not an actual act of making a will. Furthermore, a valid Islamic will has recognized methods for its proof, such as writing and testimony, and nothing of these methods was mentioned in the question. Therefore, there is no sin upon whoever inherited this land in selling it.