The Mukhya Upaniṣads

Īśa · The Whole World in the Light of the Self

Mantra 9

अन्धं तमः प्रविशन्ति येऽविद्यामुपासते ।ततो भूय इव ते तमो य उ विद्यायां रताः ॥ ९ ॥

In plain words

Into blinding darkness enter those devoted to ritual action alone; into darkness seemingly deeper still enter those delighting in meditation alone.