Touhg tough river decision with nut flush

[famousanus03]

opponent is unknow never played with him...he has check called me down twice on flop and turn then folded river..on to the hand
3 people in the hand 3 limpers and sb folded so 21 in pot
hero is dealt A2 hearts

flop is 8 10 7 2 hearts
1st gy leads 21 villain calls i call

turn is 5 hearts 1st gy leads pot (84) again villain calls i make it 270 1st gy folds villain calls

river 7 villain leads 700 all in i cover
i folded...what would u have done???