Musannaf Ibn Abi Shayba 19:228

Abu Bakr narrated to us, he said: Ibn Mahdi narrated to us, from Sa’id ibn Zayd, from Wasil, the freed slave of Abu ‘Uyaynah, from Abu ‘Uyaynah ibn al-Muhallab, he said: I heard ‘Umar ibn ‘Abd al-‘Aziz saying: Whoever confesses after he has been struck a single lash, he is a liar.