What is the means for constantly holding on to the thought ‘Who am I?’

When other thoughts arise, one should not pursue them, but should inquire:
‘To whom do they arise?’

It does not matter how many thoughts arise. As each thought arises, one should inquire with diligence,
“To whom has this thought arisen?”.
The answer that would emerge would be “To me”.
Thereupon if one inquires “Who am I?”,
the mind will go back to its source;
and the thought that arose will become quiescent.
With repeated practice in this manner, the mind will develop the skill to stay in its source.

When the mind that is subtle goes out through the brain and the sense-organs, the gross names and forms appear;
when it stays in the heart, the names and forms disappear.

Not letting the mind go out, but retaining it in the Heart is what is called “inwardness” (antar-mukha).
Letting the mind go out of the Heart is known as “externalisation” (bahir-mukha). Thus, when the mind stays in the Heart, the ‘I’ which is the source of all thoughts will go, and the Self which ever exists will shine.

Whatever one does, one should do without the egoity “I”. If one acts in that way, all will appear as of the nature of Siva (God).

~ Who Am I? (Nan Yar?)
The Teachings of Bhagavan Sri Ramana Maharshi