Per the [Star Wars: Absolutely Everything You Need to Know][1] factbook

> "R2-D2 talks in binary, a language of beeps and whistles"
>
> [![enter image description here][2]][2]

It's also mentioned in the junior novelisations for Force Awakens and the prequel novel along with one of the books in the Star Wars: Rebels series.

> *The lead TIE cracked, then flared, then just wasn’t there anymore. He
> broke to his right, yanking back on the stick. BB-8 sang a burst of
> binary, and then Poe was behind a second TIE.*
>
> <sub><sup>[Before the Awakening][3]</sub></sup>

and

> *She grabbed her staff from the walker, then ran out into the desert,
> heading toward the sound. It repeated, the same tone at a precise
> rate. Binary, from the sound of it. The language of droids.*
>
> <sub><sup>[Force Awakens: Junior novelisation][4]</sub></sup>

and

> *Ezra spun around. Kanan stood in the doorway with Hera and Chopper
> behind him. The droid snickered in binary. Ezra frowned. He should’ve
> known better than to wander around with the droid in the ship. Chopper
> had ratted him out. And Kanan did not seem happy to find him in his
> quarters.*
>
> <sub><sup>[Star Wars: Rebels - The Rebellion Begins][5] </sub></sup>


  [1]: http://starwars.wikia.com/wiki/Star_Wars:_Absolutely_Everything_You_Need_to_Know
  [2]: https://i.sstatic.net/86Gobm.png
  [3]: http://starwars.wikia.com/wiki/Before_the_Awakening
  [4]: http://starwars.wikia.com/wiki/Star_Wars:_The_Force_Awakens_(junior_novelization)
  [5]: http://starwars.wikia.com/wiki/The_Rebellion_Begins