זוהר שמות — דף קפד ע”ב

When Israel blows the shofar, the sound pierces the heavens until the mighty rock rises toward mercy. This concealing rock rises to accuse, but when it hears the sound it is subdued. On Rosh Hashanah there is judgment upon the world and the Holy One Blessed Be He sits in judgment. Israel awakens mercy through the shofar blowing, and thus judgment is transformed into mercy. That is why this day is called Rosh Hashanah and also Yom HaDin — the Day of Judgment.