Sanskrit
From the Rigveda until the time of Pāṇini (fourth century BCE) the development of the early Vedic language can be observed in other Vedic texts: the Samaveda, Yajurveda, Atharvaveda, Brahmanas, and Upanishads. During this time, the prestige of the language, its use for sacred purposes, and the importance attached to its correct enunciation all served as powerful conservative forces resisting the normal processes of linguistic change. However, there is a clear, five-level linguistic development of Vedic from the Rigveda to the language of the Upanishads and the earliest sutras such as the Baudhayana sutras.

Q: When was the time of Panini?
fourth century BCE