This tale is one of the most frequently found. The earliest account is said to be that of the Acts of Silvester. Some of the many who repeat it are Ephraem, Cedrenus, Zonaras. The following account is mainly from Glycas, p. 461–462.
When Constantine was fighting against Maxentius, after he had seen the sign of the cross, he was victorious. Then, forgetting, he was conquered, and grieving, he fell asleep and had a vision in which the blow of a switch on his nostrils brought blood which flowed down on his linen tunic in the form of a cross. Seeing this, he was filled with penitence, and became again victorious. Being led away a second time into idolatry through his wife Fausta, he was divinely afflicted with leprosy. The priests prescribed a bath in the blood of infants, and it was ordered; but when he heard the lamentations of the mothers, he said it was better to suffer than that so many infants should perish. Therefore the apostles, Peter and Paul as some say, appeared to him and told him Silvester would cure him, as he did. There are many varieties of the story and various details as to baptism, but in general the whole series of stories regarding his baptism at Rome centers in this story, and gratitude for this cure is the supposed occasion of the famous donation of Constantine. In this the circumstances of the miracle are given at length,—the words of the apostles, Silvesters identification of them as apostles by portraits, the immersion, and subsequent instruction.