Where did Maimonides learn specific Issurey Biah from?

Question

In the Mishnneh Torah, Issurei Biah 1:13 there is a law that does not punish a man who has had forbidden sexual relations with a girl who is under three years of age:

"כָּל אִשָּׁה אֲסוּרָה מֵאֵלּוּ אִם הָיְתָה בַּת שָׁלֹשׁ שָׁנִים וְיוֹם אֶחָד וָמַעְלָה גָּדוֹל הַבָּא עָלֶיהָ חַיָּב מִיתָה אוֹ כָּרֵת אוֹ מַלְקוֹת וְהִיא פְּטוּרָה מִכְּלוּם אֶלָּא אִם כֵּן הָיְתָה גְּדוֹלָה. וְאִם הָיְתָה פְּחוּתָה מִזֶּה הֲרֵי שְׁנֵיהֶן פְּטוּרִין שֶׁאֵין בִּיאָתָהּ בִּיאָה"

"When an adult male enters into relations with any of the women forbidden in connection with the above transgressions who is three years and one day old or more, he is liable for execution, kerait, or lashes and she is not liable unless she is past majority. If she is younger than this, both participants are not liable, for the act is not considered as sexual relations" (Translated by Rabbi Eliyahu Touger, Moznaim, New York 1994, p. 20).

Where did Maimonides learn this law from?

Answer 1

@Yishai's comment Mishnah_Niddah.5.4:

Rambam says (regarding 3-12 yo):

"כָּל אִשָּׁה אֲסוּרָה מֵאֵלּוּ אִם הָיְתָה בַּת שָׁלֹשׁ שָׁנִים וְיוֹם אֶחָד וָמַעְלָה,
גָּדוֹל הַבָּא עָלֶיהָ חַיָּב מִיתָה אוֹ כָּרֵת אוֹ מַלְקוֹת וְהִיא פְּטוּרָה"

He learns it from:

בַּת שָׁלשׁ שָׁנִים וְיוֹם אֶחָד, מִתְקַדֶּשֶׁת בְּבִיאָה. ... וְחַיָּבִין עָלֶיהָ מִשּׁוּם אֵשֶׁת אִישׁ...
בָּא עָלֶיהָ אַחַד מִכָּל הָעֲרָיוֹת הָאֲמוּרוֹת בַּתּוֹרָה, מוּמָתִין עַל יָדָהּ, וְהִיא פְטוּרָה.

A girl who is three years and one day old... a man other than her husband is liable for engaging in intercourse with her due to violation of the prohibition against intercourse with a married woman. ... if one of all those with whom relations are forbidden, engaged in intercourse with her, they are executed by the court for engaging in intercourse with her, and she is exempt, because she is a minor.

Next law (under 3yo):

" וְאִם הָיְתָה פְּחוּתָה מִזֶּה הֲרֵי שְׁנֵיהֶן פְּטוּרִין שֶׁאֵין בִּיאָתָהּ בִּיאָה"

He learns straight from (ibid):

"פָּחוֹת מִכָּן, כְּנוֹתֵן אֶצְבַּע בָּעָיִן:"

If the girl is less than that age, younger than three years and one day, the status of intercourse with her is not that of intercourse in all halakhic senses; rather, it is like placing a finger into the eye. (explanation: Just as in that case, the eye constricts, sheds tears, and then returns to its original state, so too, in a girl younger than three years and one day old, the hymen returns to its original state.)

A very explicit Mishna IMHO.