Per last post, here is a quote from D.M. McIntyre commenting on the life of Martin Boos:
Before the great revival in Gallneukirchen broke out, Martin Boos spent hours and days and often nights in lonely agonies of intercession. Afterwards, when he preached, his words were as flame, and the hearts of the people as grass.
Andrew A Bonar writes,
O brother, pray; in spite of Satan, pray; spend hours in prayer; rather neglect friends than not pray; rather fast, and lose breakfast, dinner, tea, and supper–and sleep too–than not pray. And we must not talk about prayer, we must pray in right earnest. The Lord is near. He comes softly while the virgins slumber.
Andrew A Bonar