Om Purnam ada purnam mantra sloka is mentioned in The Isha Upanisad. Everything animate or inanimate that is within the universe is controlled and owned by the Lord. One should therefore accept only those things necessary for himself, which are set aside as his quota, and one should not accept other things, knowing well to whom they belong.
ॐ पूर्णमदः पूर्णमिदं पूर्णात्पुर्णमुदच्यते
पूर्णस्य पूर्णमादाय पूर्णमेवावशिष्यते ॥
ॐ शान्तिः शान्तिः शान्तिः ॥
Om Shaantih Shaantih Shaantih ||
1: Om, That (Outer World) is Purna (Full with Divine Consciousness); This (Inner World) is also Purna (Full with Divine Consciousness); From Purna comes Purna (From the Fullness of Divine Consciousness the World is manifested) ,
2: Taking Purna from Purna, Purna Indeed Remains (Because Divine Consciousness is Non-Dual and Infinite).
3: Om Peace, Peace, Peace.