Bhagavan Ramana Maharshi's Heart Cave Sutra

हृदयकुहरमध्ये केवलम् ब्रह्ममात्रम् 
ह्यहमिति साक्षादात्मरूपेण भाति ।
हृदिविश मनसा स्वम् चिन्वता मज्जता वा,
पवनचलनरोघादात्मनिष्ठो भवत्वम् ।।

In the interior of the cavity of the Heart, the One Supreme Being is ever glowing with the self-conscious emanation 'I,I'. 
To realize Him, enter the Heart with one-pointed mind, by quest within, or diving deep, or control of breath and abide with the Self of self.

 - Bhagavan Sri Ramana Maharshi

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