Rashi on Leviticus 22:28/רש"י על ויקרא כ״ב:כ״ח

1אתו ואת בנו [AND WHETHER IT BE AN OX OR A SHEEP YE SHALL NOT SLAUGHTER] IT AND ITS YOUNG [BOTH IN ONE DAY] — This law applies only to the female parent, although Scripture uses the masculine term אתו — that it is forbidden to slaughter the dam and its male or female young in one day, but it does not apply to the male parent, and it is permissible to slaughter the father animal and its young whether male or female in one day (Chullin 78b; cf. also Onkelos).

2אתו ואת בנו IT AND ITS YOUNG — The prohibition of slaughtering the young first and then it (the dam) in one day is also implied (Chullin 82a).