Maliks Muwatta Book 41, Hadith Number 18.
Malik related to me from Zurayq ibn Hakim al-Ayli that a man called Misbah asked his son for help and he thought him unnecessarily slow. When the son came, his father said to him, “O fornicator.” Zurayq said, “So the son asked me to help him against the father. When I wanted to flog him, his son said, ‘By Allah, if you flog him, I will acknowledge that I have committed fornication.’ When he said that, the situation was confused for me, so I wrote about it to Umar ibn Abd al-Aziz who was the governor at that time, and I mentioned it to him. Umar wrote me to permit his pardon.”
Zurayq said, “I wrote to Umar ibn Abd al-Aziz also, ‘What do you think about a man who is slandered or his parents are slandered and both or only one of them are dead?’ He said, Umar wrote to me, ‘If he forgives, his pardon is permitted for himself. If his parents are slandered and one or both of them are dead, take the judgement of the Book of Allah for it unless he wants to veil it.'”
Yahya said, “I heard Malik say, ‘That is because the slandered man might fear that if that is unveiled about him, a clear proof might be established. If it is according to what we have described, his pardon is permitted.”