אָ֣ז אָ֭מַרְתִּי הִנֵּה־בָ֑אתִי בִּמְגִלַּת־סֵ֝֗פֶר כָּת֥וּב עָלָֽי׃
Then said I: 'Lo, I am come with the roll of a book which is prescribed for me;
Rashi on Psalms
Then at the time of the giving of the Torah, behold I came to You to be bound in Your covenant. (Exod. 24:7): “We will do and hear,” and this matter is written as testimony concerning me in the scroll of the book, i.e., in the Law of Moses.