Jeremiah 51:63

And it shall be, when thou hast made an end of reading this
book
To the captive Jews; and having also said the above words by way of prayer and approbation: [that] thou shalt bind a stone to it, and cast it into the midst of
Euphrates;
a river by which Babylon was situated. The book, being read, was to be rolled up again, and then a stone tied to it, and cast into the middle of the river, where the waters were deepest, and from whence it could not be taken up; and this was a sign confirming the above prophecy; compare with this what was done by a mighty angel concerning mystical Babylon, in which there is an allusion to this, ( Revelation 18:21 ) .