This is from St. Gregory the Great Dialogues, Book II the Life of St. Benedict: Chapter 33, A Miracle Wrought By Sister Scholastica
GREGORY: Who is there, Peter, in this world, that is in greater favor with God than St. Paul was: who yet three times desired our Lord to be delivered from the sting of the flesh, and obtained not his petition? Concerning which point also I must needs tell you, how there was one thing which the venerable father Benedict would have done, and yet he could not. For his sister called Scholastica, dedicated from her infancy to our Lord, used once a year to come and visit her brother. To whom the man of God went not far from the gate, to a place that did belong to the Abbey, there to give her entertainment.
And she coming thither on a time according to her custom, her venerable brother with his monks went to meet her, where they spent the whole day in the praises of God and spiritual talk: and when it was almost night they supped together, and as they were yet sitting at the table, talking of devout matters, and darkness came on, the holy Nun his sister entreated him to stay there all night, that they might spend it in discoursing of the joys of heaven. But by no persuasion would he agree to that, saying that he might not by any means tarry all night out of his Abbey.
At that time, the sky was so clear that no cloud was to be seen. The Nun, receiving this denial of her brother, joining her hands together, laid them upon the table: and so, bowing down her head upon them, she made her prayers to almighty God: and lifting her head from the table, there fell suddenly such a tempest of lightning and thundering, and such abundance of rain, that neither venerable Benedict, nor his monks that were with him, could put their head out of door: for the holy Nun, resting her head upon her hands, poured forth such a flood of tears upon the table, that she drew the clear air to a watery sky, so that after the end of her devotions, that storm of rain followed: and her prayer and the rain did so meet together, that as she lifted up her head from the table, the thunder, so that in one and the very same instant, she lifted up her head and brought down the rain.
The man of God, seeing that he could not by reason of such thunder and lightning and great abundance of rain return back to his Abbey, he began to be heavy and to complain of his sister, saying: "God forgive you, what have you done?" to whom she answered: "I desired you to stay, and you would not hear me, I have desired our good Lord, and he hath vouchsafed to grant my petition: wherefore if you can now depart, in God's name return to your monastery, and leave me here alone."