מַשָּׂא דְבַר ה' אֶל יִשְׂרָאֵל בְּיַד מַלְאָכִי
The burden of the word of G-d to Israel in the hand of Malachi.
What was G-d’s word doing in Malachi’s hand? To explain this, Rashi quotes a Midrash that the souls of all future prophets stood at Mount Sinai, where they were given their prophecies. They were not, however, permitted to reveal them until their proper time. Therefore, Malachi’s prophecy remained “in his hand” until it was time to share it with the nation.