Musnad Ahmad 1013:80

Ahmad ibn al-Hajjaj narrated to us, Abdullah narrated to us, Mus'ab ibn Thabit informed us, Abu Hazim narrated to me, he said: I heard Sahl ibn Sa'd al-Sa'idi narrating from the Prophet, may Allah bless him and grant him peace, he said: "Indeed, 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."