Musnad Ahmad 31:385

Yunus narrated to us, Laith narrated to us, from Yazid ibn Abi Habib, from Abu al-Nadr, that Abu Sa’id al-Khudri was complaining of his leg. His brother entered upon him while he had placed one of his legs over the other while lying down, so he struck his aching leg with his hand, causing him pain. He said, "You have caused me pain. Do you not know that my leg is aching?" He said, "Yes." He said, "Then what prompted you to do that?" He said, "Have you not heard that the Prophet, peace and blessings of Allah be upon him, has forbidden this?"