Sahih Muslim
The Book of Virtue, Good Manners and Joining of the Ties of Relationship (Kitab Al-Birr was-Salat-I-wal-Adab)
Book Number: 032 / Hadith Number: 6260
Nu'man b. Bashir reported Allah's Messenger (may peace be upon him) as saying: The believers are like one person; if his head aches, the whole body aches with fever and sleeplessness.