Musannaf Ibn Abi Shayba 19:164

Abu Bakr narrated to us, he said: Abu Usama narrated to us, he said: Jarir ibn Hazim narrated to me, he said: I read the letter of Umar ibn Abd al-Aziz to Adi ibn Artah: "As for what follows, you wrote asking about the slave who slanders a free person, how many lashes he should receive. You mentioned that it reached you that I used to lash [them] forty lashes when I was in Medina, then I lashed [them] eighty lashes at the end of my term of office. And that my first lashing was an opinion I held, and that my latter lashing conformed to the Book of Allah. So lash him eighty lashes."