A pencil and a slip of paper.† Then hand key it back in. Better yet, write it in a physical address book, not easy to erase the whole thing by accident, or by virus, or ransomware.
† (This is so you don’t forget the number you just wrote down in the era where everyone has the memory of a goldfish.)
P.S. I noticed a new thing in the old stationery store in my town (that still exists), next to little address books the same publisher added “password books”. Instead of name they put “Website:” and one of the lines for Notes they changed to “Password”.