'The Buddha Speaks on Om Sutra'

[plwk]

I was in a Dharma class last week when someone commented that Mahayana/Vajrayana Buddhists use the syllable 'Om' in dharanis/mantras like the Hindus.
Then I recalled some kalpas back when E-Sangha was up, there was a thread in the Mahayana Forum where someone raised the same issue and some user appended one link that has a purported Sutra where the Buddha reportedly taught on the syllable 'OM'. I regretted not saving that link.
Does anyone here recall or know of that link?

Many thanks in advance

[some1]

I am not sure if this is same as what was discussed at E-sangha previouly, however, most references I found are pointing to 守護國界主陀羅尼經 (Dhāraṇī for Safeguarding Nation, Realm, and Ruler) http://www.cbeta.org/result/normal/T19/0997_009.htm

Basically, "Om" is explained as a combination of "a", "u", and "m" which signify the trikaya of the Buddha. (善男子陀羅尼母所謂 唵字所以者何 三字和合為唵字故 ....)