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.
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.