מדרש על עמוס 3:1

Similarly, (Amos 3:1) "Hear this prophecy which the L-rd spoke, etc." Now did Amos prophesy only these? Is it not written (Amos 1:1) "The words of Amos, one of the herders of Tekoa, who saw visions concerning Israel"? The intent (of "this prophecy") is that they were words of rebuke, viz. (Ibid. 4;1) "Hear this thing, you cows of Bashan on Mount Shomron, etc." The "cows of Bashan" are their (corrupt) judges.