Constitution of India
Article 346 :
Official language for communication between one State and another or between a State and the Union :
The language for the time being authorised for use in the Union for official purposes shall be the official language for communication between one State and another State and between a State and the Union:
Provided that if two or more States agree that the Hindi language should be the official language for communication between such States, that language may be used for such communication.
