From where (book or lecture ) you have quoted it.
If somebody is quoting sometning, the source of quotation should also be mentioned. I say so for two reasons:
1. Whether quotation is authentic or not.
2. In what context, the statement has been made. It may happen that the quote has been made in some other context or we are quoting in some other context.
I wanted to comment on the quotation, but before that I wanted to know the above two facts.