Is there any rule for chanting Mahamantra :-
Oṃ namo bhagavate Vāsudevāya
Or can any one chant this anywhere at anytime (while travelling, eating, sleeping, resting on bed) as we can do nama smarana(taking name of lord).
Who can chanta the mantra
Any Sincere Vaishnava can chant it regardless of caste creed or sex
The mantra Om Namo Bhagwate Vasudevaya is very first verse of Shrimad Bhagwatam, Shrimad Bhagwatam is a smriti, so there are no restrictions on who is eligible to recite.
The mantra is open to all, and anyone who is sincere Vaishanva can recite(do japa) it.
Or can any one chant this anywhere at anytime (while travelling, eating , Slipping , resting on bed) as we can do namasmaran.
The basic rules of japa must be followed, Vaishnava means one who has tan(body),mana(mind), atman(soul) everything dedicated or surrendered to Shriman Narayana.
A Vaishnava must maintain sattva, it can be maintained by sattvik food and everyday life discipline is must.
Everyday life discipline includes a specific place to chant(clean, neither too hot nor too cold, noise free zone) must be chosen, an asana of choice mostly kusha asana.
Bodily discipline basic sanitation, regular bathing, and achmana, celibacy, and following Dharma as in Dharma Shastras.
For travelling only bodily discipline is enough(while travelling even Dwijas are exempted from doing Sandhyavandanam).
Resting on bed : no, sleeping no, eating: yes(if you follow food discipline and eat prasadam, then food is ambrosia)
One of the powerful mantras of Vishnu is the dwadashaksra mantra or the
Om namo bhagavate vasudevaya mantra.
It can be taken only from a Guru. One can't simply start chanting it by taking it from a book. That is a sin as per scriptures. And, this is valid for not just for this mantra but for every other mantras too.
The Vaisnava Agama shastras provide the following procedures for chanting this mantra:
Atha VAsudeva Mantrah : Pranavo Hritbhagavate VAsudevAya Kirtitah |
PradhAne Vaisha Ve Tantre Mantrohaya SurapAdapah |
Om namo bhagavate vAsudevaya, this 12-lettered mantra is "kalpapAdapa-swarupa" that is, japa of this mantra fulfills all desires of the sadhaka.
Then the procedure for pithanyasa, rishyadi nyasa, kara-anga nyasas are laid down:
Yatha sirasi prajapataye rishaye namah, mukhe gayatricchandase namaha, hrudi vAsudevAya devatayai namaha.
The above rishyadi nyasas give the rishi, chanda and devata of the mantra as prajapati , gayatri and vasudeva respectively.
Then comes kara (hands) and anga (parts of body) nyasas which are done like this ( I am intentionally omitting almost all mantras & giving only a few as samples as the full procedure should only be learned from the GUru) :
Om angushtabhyAm namha |
namastarjanibhyAm swAhA |
bhagavate madhyamAbhyAm vashata etc etc...
Then the sadhaka has to do anga nyasa with prescribed mantras given in Agamas.
After that comes mantra nyasa, where the devotee plants the 12 letters of the mantra on different parts of his body using prescribed mudras and mantras.
After that comes the dhyana part where the devotee chants the dhyana sloka for Vasudeva. Thereafter manasa puja is done by the sadhaka.
After that the sadhaka has to do pitha puja with Vishnu mantras and etc.
This mantra's purascharana is done by 12 lakh japas and 12,000 homas.
Asya Purascharanam DwAdasalakshyajapaha
Although after completing one lakh japa only a devotee attains jitendriyata or victory over senses.
Varna Laksham Japenmantram Dikshitovijitendriyoho .
Also, note that in the above verse its stated about the dikshito or one who is initiated. Now this clearly indicates that only initiated person is allowed to chant the mantra.
In any case, pranava or OM should not be chanted without initiation. See this answer for example.
So, in short, many rules are there for chanting this mantra and for every other mantras. And if you are genuinely interested in learning them , then initiation is the only method.
Source - The verses from Vaishnava Agamas as quoted in the compendium Brihat Tantrasara.
A mantra should be taken from a Guru. If you read it and started chanting is good but dont stop there. Keep chanting it but meanwhile seek some good Guru who can give you an authentic diksha of this mantra. There are rules of place, hygiene time etc. for japa on a rosary but there are no rules for mansik japa(reciting mantra in mind only without using a rosary). A japa is compared to a Yagya(sacred fire). God says in Geeta that '
'Yagyanam japayagyo asmi'
Japa is a Yagya where the one who chants burns his Agyana and Papa Karmas.
If you wish to experience the effects of japa do an Anushthana. Multiply the number of letters in a mantra by 1 lac. You will get a number. Chanting the mantra for that much time completes the Anushthana.
One should take a saltless diet during an Anushthana ,must sleep on a floor, observe silence and follow celibacy.
Hope this helps. May lord Vishnu bless us! :)