Tosefta for Horayot 3:3
חָטְאוּ עַד שֶׁלֹּא נִתְמַנּוּ וְאַחַר כָּךְ נִתְמַנּוּ, הֲרֵי אֵלּוּ כְהֶדְיוֹט. רַבִּי שִׁמְעוֹן אוֹמֵר, אִם נוֹדַע לָהֶם עַד שֶׁלֹּא נִתְמַנּוּ, חַיָּבִין. וּמִשֶּׁנִּתְמַנּוּ, פְּטוּרִין. וְאֵיזֶהוּ הַנָּשִׂיא, זֶה הַמֶּלֶךְ, שֶׁנֶּאֱמַר (ויקרא ד) וְעָשָׂה אַחַת מִכָּל מִצְוֹת ה' אֱלֹהָיו, נָשִׂיא שֶׁאֵין עַל גַּבָּיו אֶלָּא ה' אֱלֹהָיו:
If they sinned before they were appointed, and then they were appointed, they are like commoners, [it being written about a Nassi (Leviticus 4:22): "If a Nassi sin" — if he sins when he is a Nassi; and so, with the anointed priest, viz. (Ibid. 3): "If the anointed priest sin" — if he sins after he is anointed.] R. Shimon says: If they were apprised (that they had sinned) before they were appointed, they are liable; if they were apprised after they were appointed, they are exempt. And who is the "Nassi" — the king, it being written (Ibid.): "And he do one of all the mitzvoth of the L rd his G d" — a nassi who has over him only the L rd his G d (i.e., a king).