Is there any proof in Qur'an or Sunnah for paying Zakah every year in the same amount, i.e. I take Zakah 2500 from one lakh this year. If I keep the same amount, is it necessary to pay Zakah in the coming years too?Somebody says it is enough to pay Zakah once in life for the said same amount.

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Whoever owns an amount that reaches the Nisaab (85 grams of gold or 595 grams of silver or a similar value) and a year passes after he possessed this Nisaab, he has to give its Zakah regardless of why he saved that amount (for expenditure, marriage or trading).
The evidences for this are the general religious texts ordering Muslims to give Zakah whenever one of them owns a Nisaab.
Now if the person gives the Zakah of his belongings this year and the same belongings still reach the Nisaab the following year, then he has to pay its Zakah again. So as long as the person owns some property that reaches the Nisaab and as long as a year passes after he owned that property, then he has to give its Zakah.
Allah knows best.