Berean Standard Bible (BSB)
They thought that Jesus was talking about actual sleep, but He was speaking about the death of Lazarus.
World English Bible (WEB)
Now Jesus had spoken of his death, but they thought that he spoke of taking rest in sleep.
Strong's King James
Howbeit{G1161} Jesus{G2424} spake{G2046}{(G5715)} of{G4012} his{G846} death{G2288}: but{G1161} they{G1565} thought{G1380}{(G5656)} that{G3754} he had spoken{G3004}{(G5719)} of{G4012} taking of rest{G2838} in sleep{G5258}.
The F.O.G *
(13) But Yeshua spoke of his death but they thought that He spoke about the slumber of sleep.
The F.O.G Message *
Young's Literal Translation
but Jesus had spoken about his death, but they thought that about the repose of sleep he speaketh.