I cannot recollect it. Sorry.
The human will, as part of the human nature, subsists in the Person of the Logos. It has no independent personality of its own. As it was the Person of the Logos who acted through His human nature, including His human will, He was not able to sin because that Person is not able to sin.
Perhaps Geerhardus Vos could assist in thinking about this: "The person works through, in, and behind the fully human nature of the Mediator. The unity of the person is not merely verbal but answers to a reality. The human nature, though it has self-consciousness and will and feeling, does not operate of itself but is at every moment the instrument of the person who bears it. It does have substance but no subsistence in itself... It is the Logos who makes use of the human nature existing in Him as its organ... the Logos is the person for the human nature, the starting point for all its thoughts and emotions. Here for us lies the mystery in the doctrine of the Mediator as God-man. We cannot explain how human understanding, human will, and human feeling were joined to and united with the person of the Son of God."
There is also the consideration that to the human nature was given the plenitude of the Spirit both in gifts and graces to equip Christ to do the work of the Mediator for us. At every moment He acted under the sanctifying operation of the Spirit in which He was conceived and separated from sin.