Calculating down in a column product of two other cells W/O changing calculated value

[abutahir]

I want enter a formula in a column where the formula take the product of any other two cells, calculate and then go down the same column calculate dynamically again the same product from the same cells .
The reference cells from where I wan to keep values will be changing but the formula cell column should not change the earlier calculated value in the upper cells. Addressing cells will be exactly the same all the time with changing values but formula cell will be going down.
Please tell me what formula shall I use in my column?