See page 875-877 and page 897 of Duties of the Heart from Feldheim Publishers, in volume 2. My own translation of what appears on these pages (so as to avoid copyright problems) is as follows: On page 897 it is written, ""Therefore, please listen, and see, and incline your ear, and forget your people and the house of your father, and arise crying out to your King [in] your days and [in] your nights, lift up your hands towards Him, and prostrate to Him [with your] face to the ground. And your eyelids should stream waters, while kneeling upon your knees.."" Towards the beginning of page 875 it says, ""And among them (the signs of love towards the Almighty), is that at night he ""falls [in prostration]"" (a Jewish euphamism for prostration and supplication).... page 877 continues ""And whoever accepts this upon himself, should practice with the ""Admonition"", he should pray it while sitting, afterwards he should preface it with songs from the known songs/psalms, or others. And afterwards he should pray the ""Entreaty"" while standing, and [then] prostrate until the end of it. And [then] kneel and say what he desires of supplications.""
There are, in fact, many references to the practice of actual prostration throughout Jewish writings... A very deep text written about the topic, one which is unfortunately equally difficult to get one's hands on, is the volume of the book HaMaspeek l'Ovdei HaShem by Rav Avraham son of Rambam where he writes around 100 pages on the practice and significance of prostration."