सत्यमेव जयते

The origin of the Satyameva Jayathe motto is a well-known mantra 3.1.6 from the Mundaka Upanishad. The mantra is as follows:

satyameva jayate nānṛtaṁ
satyena panthā vitato devayānaḥ
yenākramantyṛṣayo hyāptakāmā
yatra tat satyasya paramaṁ nidhānam^[3]

सत्यमेव जयते नानृतं
सत्येन पन्था विततो देवयानः ।
येनाक्रमन्त्यृषयो ह्याप्तकामा
यत्र तत् सत्यस्य परमं निधानम् ॥६॥

In English

Truth alone triumphs; not falsehood.
Through truth the divine path is spread out
by which the sages whose desires have been completely fulfilled, reach
where that supreme treasure of Truth resides.

In case you didn't know, Pandit Madan Mohan Malaviya is the one who adopted this upanishad quote for the Ashoka Chakra.

More details: http://en.wikipedia.org/wiki/Satyameva_Jayate