Rashi on Numbers 7:21/רש"י על במדבר ז׳:כ״א

1פר אחד ONE BULLOCK, [A YOUNG ONE, בן בקר] — in allusion to Abraham of whom it states, (Genesis 18:7) “And he took a young bull (בן בקר)" (Numbers Rabbah 13:14).

2איל אחד ONE RAM — in allusion to Isaac, with reference to whom Scripture states, (Genesis XXII 13) “and he (Abraham) took the ram (האיל) [and offered it up … in the stead of his son]” (Numbers Rabbah 13:14).

3כבש אחד ONE LAMB — in allusion to Jacob of whom Scripture states, (Genesis 30:40) “and Jacob did separate the lambs” (Numbers Rabbah 13:14).