Chasidut for Psalms 40:8

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;