18 It is useful to note here that this very important letter of St. Leo to Flavian concerning the Incarnation of the Word was read in the third session of the council, and hardly had the voice of the reader ceased, when there went up a unanimous cry: 'This is the faith of the Fathers, this is the faith of the Apostles. So we all believe, and so believe all orthodox Christians. Let him be anathema who does not believe this. Peter has spoken through Leo' (Schwartz, II, Vol. I, pars altera, p. 81 [Act. III]; Mansi vi, 871. [Act. II])
Source: Sempiternus Rex Christus (Vatican.va)