Manava Dharmashastra 2.85

2.85

vidhiyajñāj japayajño viśiṣṭo daśabhir guṇaiḥ
upāṃśuḥ syāc chataguṇaḥ sāhasro mānasaḥ smṛtaḥ //

An offering, consisting of muttered prayers, is ten times more efficacious than a sacrifice performed according to the rules (of the Veda); a (prayer) which is inaudible (to others) surpasses it a hundred times, and the mental (recitation of sacred texts) a thousand times.