John Gill Commentary Mark 15:35

"And some of them that stood by, when they heard it, said, Behold, he calleth Elijah." — Mark 15:35 (ASV)

And some of them that stood by
The cross:

when they heard [it] ;
the loud voice of Jesus, and the words he uttered:

said, behold he calleth Elias ;
whom they ignorantly, or wilfully took for Eloi; (See Gill on Matthew 27:47).