Musannaf Ibn Abi Shayba 19:798

Abu Bakr narrated to us, he said: Sufyan ibn ‘Uyaynah narrated to us, from Humayd al-A’raj, from Yahya ibn ‘Abd Allah ibn Sayfi, that ‘Umar wrote to Abu Musa: "Do not exceed thirty [lashes] in discretionary punishment (ta’zir)."