Constitution of India, 1949
246.Official language for communication
between one State and another or between a State and the Union.-
The language for the time being authorized 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.