23 But I will not make use of the voice of evil spirits in support of the martyrs. Their holy sufferings are proved by the benefits they confer. These have persons to judge of them, namely, those who are cleansed, and witnesses, namely, those who are set free. That voice is better than that of devils, which the soundness of those utters who came infirm; better is the voice which blood sends forth, for blood has a loud voice reaching from earth to heaven. You have read how God said: “Your brother's blood cries unto Me.” This blood cries by its colour, the blood cries by the voice of its effects, the blood cries by the triumph of its passion. We have acceded to your request, and have postponed till today the deposition of the relics which was to have taken place yesterday.
Source: Letters (New Advent)