J.C. Ryle on the importance of practicing prayer, not just preaching its value:

It was said by an old writer that Luther’s habits of private prayer, and John Bradford’s habits of private prayer, were things more talked of than practised and imitated. Private prayer is one grand secret of the strength of the ministry. It is here that the roots of the ministry, practically speaking, are to be found. The ministry of a man that has gifts, however great, but who does not give the closet the principal place, must sooner or later become jejune and ineffective.

Quoted in Iain Murray, J.C. Ryle: Prepared to Stand Alone, 83.