From Power Through Prayer by Edward M. Bounds, chapter 4, Tendencies to be Avoided.
It is impossible for the preacher to keep his spirit in harmony with the divine nature of his high calling without much prayer. That the preacher by dint of duty and laborious fidelity to the work and routine of the ministry can keep himself in trim and fitness is a serious mistake. Even sermon-making, incessant and taxing as an art, as a duty, as a work, or as a pleasure, will engross and harden, will estrange the heart, by neglect of prayer, from God. The scientist loses God in nature. The preacher may lose God in his sermon.