Beauty ＆ Pure 41

The mind should discontinue its habit of perpetually moving outwards in an unlimited way, pointlessly enquiring ‘Who are you?’ and ‘Who is he?’ It should turn within, holding itself as the object [of attention] and without interruption zealously enquire ‘Who am I?’ This alone will confer the ultimate benefit.
Guru Vachaka Kovai v 384

When one unceasingly enquires ‘Who am I?’ with an acute intellect, the body-ego completely perishes through the attention that penetrates to the centre of oneself. There, reality will rise and flourish as ‘I-I’, terminating the differences that, like the azure blue of the sky, are mere appearances.
Guru Vachaka Kovai v 385

By means of the question, ‘Who is the one who questions?, all the questions that one asks, which arise through duality, will die at their very source. That question, ‘Who is the questioner?’, becoming the invincible Brahmastram, will obliterate the appearance of ‘otherness’ that manifests in the darkness of ignorance.
Guru Vachaka Kovai v386

The word ‘Brahmastram’, meaning a divine weapon of the gods, was a phrase used by some devotees to describe Bhagavan’s habit of responding to questions by saying, ‘Who is asking the question?’ The name originated with Ganapati Muni and Kapali Sastri:
Kapali Sastri: Whenever a question is put to you, you say ‘Know first who it is to whom the doubt occurs’, ‘Does anybody doubt the doubter?’, ‘Know yourself before you proceed to speak of others’, etc. This is a veritable Brahmastra [supreme weapon] at your hands to deal with the questioner…