Reference for Bekhorot 6:11
זְנַב הָעֶגֶל שֶׁאֵינָהּ מַגַּעַת לָעַרְקוֹב, אָמְרוּ חֲכָמִים, כָּל מַרְבִּית הָעֲגָלִים כֵּן, כָּל זְמַן שֶׁהֵן מַגְדִּילִין הֵם נִמְתָּחוֹת. לְאֵיזֶה עַרְקוֹב אָמְרוּ, רַבִּי חֲנִינָא בֶּן אַנְטִיגְנוֹס אוֹמֵר, לָעַרְקוֹב שֶׁבְּאֶמְצַע הַיָּרֵךְ. עַל אֵלּוּ מוּמִין שׁוֹחֲטִין אֶת הַבְּכוֹר, וּפְסוּלֵי הַמֻּקְדָּשִׁין נִפְדִּין עֲלֵיהֶן:
The tail of a calf which does not reach the leg joint - the Sages said: all growing calves are such [that their tails reach their knee joint, and] while they are growing, their tails [also] extend. To which leg joint did they refer? Rabbi Chaninah ben Antignos says, to the joint in the middle of the thigh. For [all] these blemishes [in this chapter], we slaughter the first born and invalid sanctified animals are redeemed because of them.