The Self, my dear, is to be seen, to be heard, to be reflected upon, to be meditated upon. (Yajnavalkya to Maitreyi — the fourfold sadhana of shravana, manana, nididhyasana.)
Brihadaranyaka Upanishad
बृहदारण्यकोपनिषद्
मूल श्लोकः
आत्मा वा अरे द्रष्टव्यः श्रोतव्यो मन्तव्यो निदिध्यासितव्यः॥
ātmā vā are draṣṭavyaḥ śrotavyo mantavyo nididhyāsitavyaḥ
BrU 2.4005