Revelation 10:5-7

5Then the angel whom I had seen standing on the sea and the land raised his right hand [to swear an oath] to heaven,

6and swore [an oath] by [the name of] Him who lives forever and ever, who created heaven and the things in it, and the earth and the things in it, and the sea and the things in it, that there will be delay no longer,

7but when it is time for the trumpet call of the seventh angel, when he is about to sound, then the mystery of God [that is, His hidden purpose and plan] is finished, as He announced the gospel to His servants the prophets.