Musannaf Ibn Abi Shayba 27:250

Ibn Fudayl narrated to us, from Layth, from Mujahid, who said: When David (Dawud) committed the sin—and his sin was only that when he saw her, he ordered her to be isolated, so he did not approach her—the two disputants came to him and climbed over the sanctuary wall. When he saw them, he stood up to them and said, "Get out from me! What has brought you to me?" They said, "We only wish to speak to you with a simple matter: 'Indeed, this is my brother; he has ninety-nine ewes, and I have one ewe, and he wants to take it from me.'" David (peace be upon him) said, "By Allah, he is more deserving of being sliced from here to here"—meaning from his nose to his chest. The man said, "This is David, he has already done it." David (peace be upon him) realized that he [the man] meant that by it, and he recognized his sin. He fell prostrate for forty days and forty nights, and his sin was written on his hand, which he would look at so as not to be heedless, until the vegetation grew around him from his tears, covering his head. After forty days, he cried out, "The forehead is sore, and the eye has dried up, and David (peace be upon him) has not had anything returned to him regarding his sin." Then it was called out to him, "Are you hungry so that you may be fed? Or naked so that you may be clothed? Or oppressed so that you may be helped?" He let out a sob that caused the vegetation around him to wither, as he did not mention his sin. At that moment, he was forgiven. When the Day of Resurrection comes, his Lord will say to him, "Be in front of Me." He will say, "O my Lord, my sin, my sin." He will say, "Be behind Me." He will say to him, "Take hold of My foot," and he will take hold of His foot.