The sloka is to be found here.
मंत्राणां पल्लवो वासो । मंत्राणां प्रणव: शिर: । शिर: पल्लव संयुक्तो । कामधुक् भवेत् ।।
The meaning: The letters or words in a mantra constitute its body and the Om at its beginning, is the head. If both the head and the body are present then with that mantra one’s aspirations are fulfilled.
By origin i mean scriptural origin.
Also, as one can see this info is related to my this question .Because this sloka clearly states that a mantra if recited without OM at its start bears no fruit.