Be sitting on a gold mine phrase
To possess something very valuable, especially without realizing it
Only when the valuer told me how precious this stone is, I realized that I was sitting on gold mine.
The land he inherited is very valuable, yet he doesn't know he is sitting on gold mine.
A proverb used to express that someone have strongly possession over something after a long time of using/keeping it. Others can not take it away from he/she, even if they have the legal rights to do so.
People tend to remember the bad things others do and while they easily forget or get rid of others' good things.