Reference for Avodah Zarah 47:7

אמר מר ישראל היו משמרין אותה משעה שנולדה וניחוש דלמא רבעי לאמא כי הוה מעברה דאמר רבא וולד הנוגחת אסורה היא וולדה נגחו וולד הנרבעת אסורה היא וולדה נרבעו אימא ישראל היו משמרין אותה משעה שנוצרה

The Master said: Israelites watched the heifer from the time of its birth. But we should be suspicious that they had bestiality with its mother when she was pregnant, for Rava said: The young of a goring cow is unfit, for it was both the cow and her young that did the goring. Likewise the young of an animal that had engaged in bestiality is unfit, since the animal and the young were ill-used together? Rather, say that Israelites watched it from the time it was first formed.

Jastrow