Jamieson, Fausset and Brown on Revelation 10:11

11. he said--A, B, and Vulgate read, "they say unto me"; an indefinite expression for "it was said unto me."

Thou must--The obligation lies upon thee, as the servant of God, to prophesy at His command.

again--as thou didst already in the previous part of this book of Revelation.

before, &c.--rather as Greek (epilaois), "concerning many peoples," &c., namely, in their relation to the Church. The eating of the book, as in Ezekiel's case, marks John's inauguration to his prophetical office--here to a fresh stage in it, namely, the revealing of the things which befall the holy city and the Church of God--the subject of the rest of the book.