The nature of water is soft, that of a rock is hard. But if a narrow-necked bottle is hung above a stone, drop by drop the water wears away the rock. So it is with the word of God: it is soft and our heart is hard. But when a person hears the word of God, often then his heart is opened to the fear of God.
The Apophthegmata Patrum (the Sayings of the Fathers) is a collection of proverbs and quotes from 4th-5th c. monks. This quote is from Everett Ferguson, Inheriting Wisdom, Readings for Today from Ancient Christian Writers. Peabody: Hendrickson, 2004, page 229.