Source of Gayatri Mantra (गायत्री मन्त्र)
The Gayatri (गायत्री) Mantra, also known as the Savitri (सवित्री ) Mantra, is a mantra from the Rig Veda (Mandala 3.62.10), dedicated to Savitr(सवितृ) also known as mother of Vedas
During its recitation, the hymn is preceded by oṃ (ॐ) and the three words – bhūr bhuvaḥ svaḥ (भूर् भुवः स्वः). This prefixing of the mantra is described in the Taittiriya Aranyaka (2.11.1-8), which states that it should be chanted with the syllable oṃ, followed by the three Mahāvyāhṛti (महाव्याहृति)and then Gayatri Mantra

Taittiriya Aranyaka (2.11.1-8)
Gayatri Mantra with English & Hindi Meaning
ॐ भूर्भुव: स्व: तत्सवितुर्वरेण्यं भर्गो देवस्य धीमहि धियो यो न: प्रचोदयात्।
ओ३म्–सर्वरक्षक परमात्मा
भूः–प्राणों के प्राण
भुवः—दुःख-त्राता
स्वः—सुखों के दाता
तत्—वह (परमात्मा)
सवितुः—प्रेरक परमात्मा
वरेण्यम्—वरण करने योग्य
भर्गः—भरण-पोषण करने वाला
देवस्य—देव का
धीमहि—हृदय में धारण करें
धियः—बुद्धियों को
यः—जो
नः—हमारे
प्रचोदयात्—प्रेरित करें ।
Aum = Brahman/Creator
bhoor = embodiment of vital energy(pran)
bhuwah = destroyer of sufferings
swaha = embodiment of happiness
tat = that
savitur = bright like sun
varenyam = best choicest
bhargo = the nourisher of all
devasya = divine
dheemahi = may hold in mind
dhiyo = intellect
yo = who
naha = our
prachodayat = may inspire