From the Golden Legend;

On a time it happed that, Saint Gregory in his cell of the same abbey whereas he was abbot wrote something, and an angel appeared to him in semblance of a mariner, which seemed as he had escaped from the tempest of the sea, and prayed him weeping to have pity on him. Then Saint Gregory commanded that there should be given to him six pence, and then he departed. The same day the angel came again in like wise as he did tofore, and said that he had lost all his good, and prayed him that he would yet help him; on whom S Gregory had yet pity, and did to be given to him six pence more, yet at the third time he came and made great cry and wept, and prayed him that he would yet help him toward his great loss, so that Saint Gregory commanded his provost that he should yet give to this poor man an alms. And the provost said that there was no more silver in all the abbey, but a dish of silver in which his mother was wont to send him pottage. And Saint Gregory commanded anon that that dish of silver should be given to him and the angel took it with great joy. And little while after, this angel appeared to Saint Gregory and said to him that God hath sent him so to him.

Saint Gregory did every day so great alms that many in the country about were nourished by him, whom he had by name written, and also the monks that dwelt in the Mount Sinai had of him their sustenance. Among all other alms that he did he governed three thousand virgins, to whom he sent every year four score pound of gold, and also he founded to them an abbey in Jerusalem, and sent to them that therein were such things as they lacked. Every day had he poor men to dinner. On a time it happed that he took the laver for to give water to a pilgrim for to wash his hands by great humility, and anon the pilgrim vanished away, whereof Saint Gregory had marvel. The night after our Lord appeared in a vision and said to him: The other days thou hast received me in my members, but yesterday thou receivedst me in my person.

Another day Saint Gregory commanded to his dispenser that he should bring to dinner twelve poor men, and when Saint Gregory and the poor men were set at meat, he told at the table sitting thirteen poor pilgrims, and demanded of his dispenser why he had done above his commandment to bring in more than twelve persons. And anon the dispenser, all abashed, went and told the poor men, and found but twelve, and said to Saint Gregory: Holy father, there be no more but twelve, and so many shall ye find and no more. Then considered Saint Gregory that, one of the pilgrims that sat next to him oft changed his visage, for oft he seemed young, and after old. And after dinner Saint Gregory took him by the hand and brought him into his chamber, and prayed him that he would tell him his name. And he answered: Wherefore demandest thou my name, which is marvellous? Nevertheless know thou well that I am the same poor mariner to whom thou gavest the dish of silver in which thy mother was wont to send the pottage, and know for certain that sith that day that thou didst to me that alms, God hath destined thee to be pope. And said moreover: I am the angel of God, and he hath sent me hither to thee to be thy defender and procurer of that which thou wouldst demand and impetre of him, and after this the angel vanished away.

http://catholicsaints.info/the-golden-legend-the-life-of-saint-gregory/