Reference for Avodah Zarah 45:6

ממאי דלמא דכ"ע לא חיישינן לרביעה והכא היינו טעמא דרבי אליעזר כדרב יהודה אמר רב דאמר רב יהודה אמר רב הניח עליהן עודה של שקין פסלה ובעגלה עד שתמשוך בה

From where [do you we know this]? Perhaps all agree that we are not concerned about bestiality, and here the reason for R. Eliezer’s opinion is like that which was said by Rav Judah in the name of Rav, for Rav Judah said in the name of Rav: [In the case of the red heifer] even if a bundle of sacks was laid on her she is disqualified, but in the case of the calf, only if she had been made to draw a burden.

Jastrow