Reference request: The area of the modular curve $mathcal{H}/SL(2,mathbb{Z})$ is $pi/3$.

I’m looking for a reference that the hyperbolic area of the modular curve $mathcal{H}/SL(2,mathbb{Z})$ is $pi/3$.

Also, what is the measure relative to which this is true? In what sense is this measure "canonical"? (ie, why do we choose this measure, and not, say, 5 times this measure?)