Purnamadah Purnamidam

ॐ पूर्णमदः पूर्णमिदं पूर्णात्पूर्णमुदच्यते । पूर्णस्य पूर्णमादाय पूर्णमेवावशिष्यते ॥
oṃ pūrṇam adaḥ pūrṇam idaṃ pūrṇāt pūrṇam udacyate | pūrṇasya pūrṇam ādāya pūrṇam evāvaśiṣyate ||

Source: Brihadaranyaka Upanishad 5.1.1; also the invocation (shanti-pathah) of the Isavasya Upanishad