i was taught that it is a property of correlation coefficient r the correlation of X with Y is the same as of Y with X

(from the course pdf file with notes  ):

LO 5. Note that correlation coecient (R, also called Pearson's R) has the following properties:

• the magnitude (absolute value) of the correlation coecient measures the strength of the linear association between two numerical variables
• the sign of the correlation coecient indicates the direction of association
• the correlation coecient is always between -1 and 1, -1 indicating perfect negative linear association, +1 indicating perfect positive linear association, and 0 indicating no linear relationship
• the correlation coecient is unitless
• since the correlation coecient is unitless, it is not aected by changes in the center or scale of either variable (such as unit conversions)
• the correlation of X with Y is the same as of Y with X
• the correlation coecient is sensitive to outliers

But since then  ,i can not find any other reference to this,that agrees with the symmetry thing... Maybe someone has some explanation?(not about intersect and slope,but about the r coefficient,? Thanks in advance

I was taught that it is a property of correlation coefficient that $r$ the correlation of $X$ with $Y$ is the same as of $Y$ with $X$.

(from the course pdf file with notes):

LO 5. Note that correlation coefficient ($R$, also called Pearson's $R$) has the following properties:

• the magnitude (absolute value) of the correlation coefficient measures the strength of the linear association between two numerical variables
• the sign of the correlation coefficient indicates the direction of association
• the correlation coefficient is always between -1 and 1, -1 indicating perfect negative linear association, +1 indicating perfect positive linear association, and 0 indicating no linear relationship
• the correlation coefficient is unitless
• since the correlation coefficient is unitless, it is not affected by changes in the center or scale of either variable (such as unit conversions)
• the correlation of $X$ with $Y$ is the same as of $Y$ with $X$
• the correlation coefficient is sensitive to outliers

But since then, I cannot find any other reference to this, that agrees with the symmetry thing. Maybe someone has some explanation.