"O thou who hast taught us that we are most truly free when we lose our wills in thine: Help us to attain to this liberty by continual surrender unto thee; that walking in the way which thou hast prepared for us, we may find our life in doing thy will; through Jesus Christ our Lord."
Attributed to the Gelasian sacramentary. Yet the closest prayer in the 8th-century Gelasian sacramentary that, working back from the English ("our wills", "liberty", "our life", "thy will"), I've found so far is no. 557 (=no. 945 in the Gelasian sacramentary of Angoulême (Paris B.N. lat 816) =no. 103 in Bruylants, =no. 1802 in Corpus orationum). For more on that one, go here.