The sentence was reported in "The Final Days", Bob Woodward and Carl Bernstein
It was said by Kissinger in front of Alexander Haig, newly appointed White House chief of staff, in Haig new office in 1973
I quote to you the sentences related: as you may notice the quote between brackets is "dumb, stupid animals to be used" that was never denied by Kissinger
In Haig's presence, Kissinger referred pointedly to military men as "dumb, stupid animals to be used" as pawns for foreign policy. Kissinger often took up a post outside the doorway to Haig's office and dressed him down in front of the secretaries for alleged acts of incompetence with which Haig was not even remotely involved. Once when the Air Force was authorized to resume bombing of North Vietnam, the planes did not fly on certain days because of bad weather. Kissinger assailed Haig. He complained bitterly that the generals had been screamin for the limits to be taken off but that now their pilots were afraid to go up in a little fog. The country needed generals who could win battles, Kissinger said, not good briefers like Haig.
On another occasion, when Haig was leaving for a trip to Cambodia to meet with Premier Lon Nol, Kissinger escorted him to a staff car, where reporters and a retinue of aides waited. As Haig bent to get into the automobile, Kissinger stopped him and began polishing the single star on his shoulder. "Al, if you're a good boy, I'll get you another one," he said.
end of quote
Bob Woodward & Carl Bernstein
The Final Days
second Touchstone paperback edition (1994)
Chapter 14, pp. 194-195