Musannaf Ibn Abi Shayba 33:191

Ali ibn Ishaq narrated to us, from Ibn al-Mubarak, from Mus'ab ibn Thabit, he said: Abu Hazim narrated to me, he said: I heard Sahl ibn Sa'd narrating from the Prophet, may God bless him and grant him peace, he said: "The believer among the people of faith is in the position of the head to the body. The believer feels pain for the people of faith just as the body feels pain for what is in the head."