John 11:11-13 NIV

11 After he had said this, he went on to tell them, "Our friend1 Lazarus has fallen asleep;2 but I am going there to wake him up."

References for John 11:11

12 His disciples replied, "Lord, if he sleeps, he will get better."
13 Jesus had been speaking of his death, but his disciples thought he meant natural sleep.3

References for John 11:13