Berean Study Bible
They thought that Jesus was talking about actual sleep, but He was speaking about the death of Lazarus.

King James Bible
Howbeit Jesus spake of his death: but they thought that he had spoken of taking of rest in sleep.

American Standard Version
Now Jesus had spoken of his death: but they thought that he spake of taking rest in sleep.

Darby Bible Translation
But Jesus spoke of his death, but they thought that he spoke of the rest of sleep.

Douay-Rheims Bible
But Jesus spoke of his death; and they thought that he spoke of the repose of sleep.

English Revised Version
Now Jesus had spoken of his death: but they thought that he spake of taking rest in sleep.

Young's Literal Translation
but Jesus had spoken about his death, but they thought that about the repose of sleep he speaketh.