John Gill Commentary Mark 11:23

"Verily I say unto you, Whosoever shall say unto this mountain, Be thou taken up and cast into the sea; and shall not doubt in his heart, but shall believe that what he saith cometh to pass; he shall have it." — Mark 11:23 (ASV)

For verily I say unto you, that whosoever shall say unto
this mountain

The Mount of Olives, at, or near which they now were.

be thou removed, and be thou cast into the sea ;

That is, of Galilee, which was nearest, and yet many miles off.

and shall not doubt in his heart, but shall believe that those
things which he saith shall come to pass ;

Not only as to removing a mountain, and casting it into the sea, but any thing equally difficult.

he shall have whatsoever he saith ;

Whatever he commands shall be done; (See Gill on Matthew 21:21).