Bava Kamma 77a — the Talmud in English

Red heifer

after its blood has been sprinkled, is

susceptible to food impurity,

even though it is forbidden to derive benefit from, and even redemption is of no avail to it after its blood has been cast, and according to Rabbi Simeon, anything that is forbidden to derive benefit from does not become susceptible to food impurity, and the reason is:

since it had a time of fitness

from when it was slaughtered, when it was fit to become susceptible to food impurity through it.