Jamieson, Fausset and Brown on Revelation 10:5

5. lifted up his hand--So A and Vulgate read. But B, C, Aleph, Syriac, and Coptic, "... his right hand." It was customary to lift up the hand towards heaven, appealing to the God of truth, in taking a solemn oath. There is in this part of the vision an allusion to Da 12:1-13. Compare Re 10:4, with Da 12:4, 9; and Re 10:5, 6, end, with Da 12:7. But there the angel clothed in linen, and standing upon the waters, sware "a time, times, and a half" were to interpose before the consummation; here, on the contrary, the angel standing with his left foot on the earth, and his right upon the sea, swears there shall be time no longer. There he lifted up both hands to heaven; here he has the little book now open (whereas in Daniel the book is sealed) in his left hand (Re 10:2), and he lifts up only his right hand to heaven.