Musannaf Ibn Abi Shayba 19:601

Abu Bakr narrated to us, he said: Marwan bin Mu'awiyah narrated to us, from 'Asim, from Abu 'Uthman, he said: A man was brought to 'Umar regarding a prescribed punishment (hadd), and a whip was brought. He said: "I want one softer than this." So a whip that had softness was brought. He said: "I want one harder than this." So a whip between the two whips was brought. He said: "Strike, and do not let your armpit be seen, and give every limb its due."