Though Father, Son, and Holy Ghost are one nature, yet do thou hold most firmly that They be Three Persons; that it is the Father alone who said, “this is my beloved Son;” the Son alone over whom that voice of the Father was heard; and the Holy Ghost alone who in the likeness of a dove descended on Christ at His baptism.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: Fulgent. de Fide ad Petrum. c. 9