Whenever we pray, we tend to keep our hands together. Is there a reason why we do so ??
Namaste is derived from Sanskrit and is a combination of two words, "Namaḥ" and "te". Namaḥ means 'bow','reverential salutation' or 'adoration' and te means 'to you'. Therefore, Namaste literally means "bow to you" translated as "I bow to you". We bow to god whenever we pray so that his blessings are on us and he fulfills the wish.– S.LAKSHMINARAYANApr 23, 2016 at 16:25
There are two verses in the Bhagavad gita that talk about that, 11.35 and 11.14. You can see it at http://www.vedabase.com/en/bg/11/35 and http://www.vedabase.com/en/bg/11/14
And the word describing this practice is kṛtāñjali, meaning "one who joins the hollowed palms in reverence or to solicit a favour (holding the hollowed palms together as if to receive alms or an offering), standing in a reverent or respectful posture".
Arjuna did it to offer obeisances to the Supreme Lord Krishna.
Today we are also doing the same because it's a common practice in Hindu dharma as we see from the above Gita verses and other scriptures.