In Hil. Teshuvah 3:7 of the Mishneh Torah (in his listing of heretics - minim), the Rambam makes the following statement:
והאומר שיש שם מנהיג אבל הן שנים או יותר
"The one who says there is indeed a [God] directing [the universe], but that there are two or more."
What is the basis of this ruling?
And where is it stated in earlier sources (pre-Rishonim) that only God has control over everything?
The shema? But seriously, see hagiga 15a. Acher's heresy was stating that metatron was a power equal to hashem. Upon seeing metatron sitting in heaven, he proclaimed "There are indeed two powers in Heaven."
The sources are vast, ranging from the Tanakh itself, through Tannaitic and Amoraic eras, and into the Geonic era (since you specifically asked for sources pre-rishonim).
The Shema (Devarim 6:4) is of course the flagship statement of Jewish monotheistic faith (see also Devarim 4:35 and others).
See also Yeshayahu 44:24 and 45:12; Tehillim 135:6; Daniel 4:35; and others.
As for the talmudic sages, see tractates Sanhedrin and Hagigah. There is alot of material here, so do some searches.
The apex that will include everything that has come before it are geonic works such as HaNivhar Emunoth wa-Deoth by Rav Saadyah HaGaon and Hovoth HaLevavoth by Rabbenu Bahya ibn Pequdah (particularly in the section Shaar HaYihud).
In short, the source for the specific ruling of the Rambam is traditionally attributed to Mishnah, Sanhedrin chapter 10. As for the rationale of his ruling, Jews have historically written about it, but have usually chalked it up to the conclusion of logic and common sense.
Hope this helps. Kol tuv.
R. Menachem Krakowski's Avodat ha-Melekh on Hilkhot Teshuvah 3:7 (also here) brings the following sources (amongst others), to support Ramabm's statement, that one who says there are two or more rulers is called a Min:
In Sanhedrin 38a, it says:
תנו רבנן: אדם יחידי נברא, ומפני מה? שלא יהו המינים אומרין: הרבה רשויות בשמים
My translation (based on the Soncino translation, and note 24 there):
Our Rabbis taught: Man was created alone. And why so? — That the Minim might not say: There are many ruling powers in Heaven.
In Sanhedrin 38b, it says:
אמר רבי יוחנן: כל מקום שפקרו המינים, תשובתן בצידן: "נעשה אדם בצלמנו" - "ויברא אלהים את האדם בצלמו", "הבה נרדה ונבלה שם שפתם" - "וירד ה' לראות את העיר ואת המגדל", "כי שם נגלו אליו האלהים" - "לאל העונה אותי ביום צרתי", "כי מי גוי גדול אשר לו אלהים קרובים אליו כה' אלהינו בכל קראנו אליו", "ומי כעמך כישראל גוי אחד בארץ אשר הלכו אלהים לפדות לו לעם","עד די כרסוון רמיו ועתיק יומין יתיב".
R. Johanan sad: In all the passages which the Minim have taken [as grounds] for their heresy, their refutation is found near at hand. Thus: "Let us make man in our image"(Bereshit 1:26) — "And God created [sing.] man in His own image"(ibid. v. 27 ); "Come, let us go down and there confound their language"(Bereshit 11:7), — "And the Lord came down [sing.] to see the city and the tower"(ibid. v. 5); "Because there were revealed [plur.] to him God"(Bereshit 35:7), — "Unto God who answereth [sing.] me in the day of my distress"(ibid. v. 3); "For what great nation is there that hath God so nigh [plur.] unto it, as the Lord our God is [unto us] whensoever we call upon Him [sing.]"(Devarim 4:7); "And what one nation in the earth is like thy people, [like] Israel, whom God went [plur.] to redeem for a people unto himself [sing.]"(Shemuel II 7:23), "Till thrones were placed and one that was ancient did sit"(Daniel 7:9).
In Bereshit Rabah 8:9, it says:
שאלו המינים את רבי שמלאי: כמה אלהות בראו את העולם?
In my translation:
The Minim asked Rabbi Simlai: How many gods created the world?
