Aham Brahma Asmi is described in the third Chapter of Tejo Bindu Upanishad.
Lord Kumara asked Lord Siva “How to attain Self-realisation?”
The reply of Lord Siva was the third chapter of the Upanishad.
How to attain Self Realisation?
I am the Brahman. I am the absolute bliss, absolute consciousness and the Supreme Being who is tranquillized, eternal and everlasting.
( “I “ here does not denote Lord Siva alone, it is used in a wider sense of Chinmatra).
Lord Siva continues.
I have no Aham-hara. I have no “I” with me. I am devoid of anything. I am the ether of consciousness or CitAkasa.
I am Turya only. (Turya is the fourth state of Atman beyond Jagrat or waking, Swapna or dreaming and Susupti or dreamless sleep).
I am the pure, absolute love, absolute knowledge and happiness. I am desire-less, disease-less, change-less and without any attachment. I am imperishable and indivisible one essence.
I am boundary-less. I have no limits and I am also the core of the core. I am beyond the scope of Speech and mind. I am the Atman or self.
I am the Paramatman or Supreme Self. I have no beginning, means or end. I resemble ether and transcending ether and transcending all. I am the residuum that ever remains. I am devoid of body, feelings and other mental functions or material perceptions. I am the inference.
I am full. I am always content and satisfied. I only appear as everything. Nothing is beyond me. I support all the three worlds. I am served by Vedas and demonstrated by Sastras. I am always fixed in Chitta meditating.
I am the absolute Brahman. I am non-dual and include everything. I am the Adi-Sesa or prime residuum and Sesa or final residuum. I have no name or form. I am beyond liberation and bondage. It is I am who is beyond Turya, I am the importance of Om.
I do not exist and I am not non-existing which is beyond the scope of mind and speech.
I am neither the mind nor the intellect nor the doubts arising. I am neither in the state of waking nor dreaming nor in dreamless sleep. I am neither subject to three miseries caused by Atman, Devas and Bhutas, nor subject to the three desires caused by wealth, Son, and ladies.
I have no dissimilarities in space, time, and substance anywhere, anytime and in anything. Except for absolute consciousness, everything is false. Vedas and Sastras, Intellect, wisdom, the body, mind, and everything is false.
Know that all truth is false. Know that Rudra, Vishnu and Brahma are false. Know that all beings are false. Know that what is perceived and seen are false. All enjoyments and sufferings are false. Good and bad or right and wrong, Victory and defeat, gain and loss, and taste and smell are all false.
I alone the absolute truth. I am Brahman only. Aham Brahma Asmi. Aham means “I”. Brahma means “Supreme God” and Asmi means “am”. I am the Supreme Being. This can also be read as Aham Brahmasmi.
Aham Brahma Asmi is the powerful Veda mantra that should be chanted with the above meditation. This is known as Atma-mantra.
This mantra destroys all the sins, all the mantras and bondage of death. It destroys the ignorance. It destroys the bondage of mind, all the diseases of the mind, intellect and body. It destroys grief, anger, lust, greed, mental functions, desires, faults, dependency, knowledge and wisdom.
It gives pleasure that cannot be spoken of. It gives bliss of knowledge that cannot be known otherwise. It will liberate all. This mantra alone is enough and no other mantra is required. There is no doubt about this.
Thus ends the third chapter of this Upanishad.
This is one of the greatest technique given in Vedas. Simply Meditate on this and Chant Aham Brahma Asmi. The Japa of this mantra and meditation with its meaning rightly understood is enough for any spiritual advancement and transformation.
Reference: Sanskrit Text Reference