What it means
"I bow to Narayana." One reading of the name is the one who rests on the waters; another is the refuge of all beings. The mantra has eight syllables, so it's known as the Ashtakshara.
Where it comes from
The Narayana Upanishad describes how the mantra is built: first Om, then namah, then narayanaya. In the Shrivaishnava tradition of South India it's called the Tirumantra, the first of three "secret" mantras. There's a well-loved story that Ramanujacharya was told to keep it private, and instead climbed the temple tower at Tirukoshtiyur and called it out to everyone below, so no one would be left without it.
How to chant it
On a tulsi mala, with a tulsi leaf placed before a picture of Vishnu. Calmly, with each syllable clear.
When: Morning suits it. Thursdays and Ekadashi especially.
Try it now
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