The Adornment of the Spiritual Marriage, by Jan van Ruysbroeck, , at sacred-texts.com
At times the man shall behold his friends in purgatory, and shall consider their misery and their yearnings and their heavy pains. Then shall he pray and beseech the pity, the mercy, and the generosity of God; and shall plead their good-will, and their great misery, and their yearning after the rich goodness of God, and he shall bring to God's remembrance that they died in love, and that their only refuge is in His passion and mercy.
Now understand this: it may sometimes happen that this enlightened man is specially urged of the Spirit of God to pray for a certain thing, for some sinner, or for some soul, or for some ghostly benefit, in such a way that he feels and understands it to be the work of the Holy Ghost, and not of his own choice, or self-will, or nature. Then the man sometimes becomes so intense and so ardent in his prayer that he receives in ghostly wise the answer that his prayer has been heard. And with the coming of this sign the thrust of the Spirit and the prayer abate.