In the triratna Buddhist community (who I practice with) there is a prostrations practice for those going through the ordination process. I'm sure we have taken this practice from another tradition.I'm fairly certain it is Tibetan but can anyone give details of the exact place in the Tibetan practice this comes from and also what is the purpose of this practice. How does it fit into the system of practice and what effects this practice has on the practicioner.
In Tibetan Buddhism, according to Chogyam Trungpa prostration is a practice of surrendering one's ego, to the triple treasure exemplified by the guru. A student full of oneself is compared to an upside down pot or else to a full cup, and prostration is seen as either turning the pot right side up or emptying the cup.
In Korean Zen, the purpose is supposedly getting one's body and mind in sync with each other, although I will be damned if my Zen Master ever explained this explicitly, you have to figure it out yourself from the various cursory remarks. Getting body and mind in sync means if your body is bowing your mind is bowing as well.
My present teacher (a modern non-sectarian master) explains bowing as the practice of sincerity, which relates both to the mindfulness of body/energy, but also to a sincere attitude towards the triple treasure. This in turn leads to the experience of selflessness and interbeing.
Some texts attribute the original authorization of prostration to the Buddha himself, although they are clear to differentiate between the Hindu Bhakti and guru worship. http://www.accesstoinsight.org/lib/authors/khantipalo/wheel206.html
However, He did lay down three forms of reverence for bhikkhus; wearing the robe with the right shoulder bared, kneeling down, and holding the palms of the hands together in the gesture of reverence. Prostration at the feet of the Buddha is also mentioned many times in the Suttas. Lay people are free to show their reverence in any suitable way and people of those times were recorded in the Suttas as expressing their reverence variously:
So the Kalamas of Kesaputta approached the Lord. Having approached him, some prostrated towards the Lord and sat down at one side; some greeted the Lord politely, and having conversed in a friendly and courteous way, sat down to one side; some raising their hands in añjali to the Lord sat down to one side, some called out their names and those of their clans and sat down to one side; while others saying nothing sat down to one side.
— Kalama Sutta, Anguttara-nikaya iii 65 (PTS edition). See A Criterion of True Religion, Mahamakut Press, Bangkok, and The Kalama Sutta, Wheel No. 8, BPS, Kandy, Sri Lanka.
And additional evidence of the Buddha accepting prostrations http://en.wikipedia.org/wiki/Prostration_(Buddhism)
In the Pali canon, laypersons prostrating before the then-living Buddha is mentioned in several suttas. In Theravada Buddhism, as part of daily practice, one typically prostrates before and after chanting and meditation. On these occasions, one does typically prostrates three times: once to the Buddha, once to the Dhamma, and once to the Sangha. More generally, one can also prostrate before "any sacred object of veneration."
But is there evidence of prostrations by religions previous to Buddhism? http://www.chayas.com/qidah.htm
Bowing, kneeling, yoshev la'areS, qidah & prostration [to the one G-D] originated in authentic Torah Teachings (the Torah of Moses). It certainly preceded Islam and Christianity. It is still practiced (to some extent) today and is/was retained by a minority of Jews, even during the Middle Ages. And while portions of this original practice may (or may not have) been copied by other religions, anyone familiar with ancient practice knows we still preserve the original method of prostration. In my opinion, a popular return to proper Jewish prostration or qidah, (which brings about focused, meaningful prayer) will result in a greater level of shifa for the Jewish people (and the world). This translates into a greater level of Divine protection (based on concepts in 'The Guide' / Moreh Nevukhim).
So there may be evidence of your particular Buddhist sect deriving the tradition, but if your question is where was prostration first introduced there is evidence that others preceded Tibetan traditions.
At a Korean Zen meditation center I visit occasionally, a practice of 108 Bows is held for special occasions with the focus being on repentance.
108 Bows of Repentance and Renewal Reflection on unwholesome states, body, speech and mind and rededication to wholesome practice to eliminate suffering through the practice of 108 full prostrations.
In fact, even more ambitious practices of hundreds and even thousands of bows are done at this center although they are split up over a period of days or weeks.
Here is a simple explanation from another Zen Center as to the purpose of this particular prostration practice of 108 bows. (source: http://www.providencezen.org/why-we-bow-108-prostrations)
We always bow one hundred and eight times. One hundred and eight is a number from Hinduism and Buddhism. That means there are one hundred and eight defilements in the mind. Or, sometimes they say one hundred and eight compartments in the mind. Each bow takes away one defilement, cleans one compartment in your mind. So our bowing practice is like a repentance ceremony every morning. In the daytime, in our sleep, our consciousness flies around somewhere. Also, we make something, we make many things in our consciousness. Then, we repent! So we do one hundred and eight bows; that’s already repenting our foolish thinking, taking away our foolish thinking.
This is a fairly strenuous practice for someone who hasn't been doing this type of activity regularly. My own "foolish thinking" that I could do them all at once without working up to it lead to quite a bit of soreness the next day. :)
Here's a video from youtube on what this looks like. Joyful, disciplined, and humbling.