tefilla - Sources for Holding One's Left Hand in Right Hand During Prayer

What are the earliest sources for the practice of holding one's left hand in one's right hand during prayer? (particular during the Ammidah).

The Talmud Shabbat (10a) notes a practice of holding one's hands during prayer, but does not specify the right hand being over the left.

רבא שדי גלימיה ופכר ידיה ומצלי אמר כעבדא קמיה מריה

I always associated this with later kabbalists, but R. Avrahan ben HaRambam writes in Sefer HaMaspik L'ovdey Hashem (Halachic portion p. 65; translated from Arabic):

ידיו שלובות ימינו על שמאלו כעבד נצב בשירות אדוניו, מנומס בעמידתו ירד וחרד

So my question is, is this the earliest known source or are there sources that can conclusively be proven to be earlier.

Update: currently the earliest source is the Mishneh Torah in the answer below. If someone posts an earlier source, the answer will be re-awarded.

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Note: if I mistranslated the Gemara, or the Hebrew HaMaspik text, or if the Hebrew text seems to be a mistranslation of the Arabic, please correct.

Note: please only cite sources which definitely predate this. E.g. Mishneh Torah whose (rough) date of authorship is undisputed.

Answer

In the Mishneh Torah, Hilkhot Tefilah 5:4 it says:

תיקון הגוף כיצד: כשהוא עומד בתפילה, צריך לכוון את רגליו זו בצד זו; ונותן עיניו למטה, כאילו הוא מביט לארץ; ויהיה ליבו פנוי למעלה, כאילו הוא עומד בשמיים; ומניח ידיו על ליבו כפותין, הימנית על השמאלית. ועומד כעבד לפני רבו, באימה ויראה ופחד. ולא יניח ידיו, על חלציו.‏

In Eliyahu Touger's translation:

The preparation of one's body: What is implied?

When one stands in prayer, he should place his feet together side by side. He should set his eyes downwards as if he is looking at the ground, and his heart upwards as if he is standing in Heaven.

His hands should be resting on his heart, with the right hand clasped over the left hand. He should stand like a servant before his master, in fear, awe, and dread. He should not rest his hand on his hips [during the Amidah].