The first mention of Krishna as the son of Devaki is found in the:

The earliest mention of "Krishna, son of Devaki" (Krishna Devakiputra) is found in the Chandogya Upanishad (3.17.6).

What the Chandogya Upanishad says:

• It mentions that the sage Ghora Angirasa taught a doctrine to "Krishna, the son of Devaki".
• After receiving this teaching, Krishna is said to have become free from thirst (desire).
• The teaching compares a person's life to a sacrifice, and stresses virtues such as austerity, charity, uprightness, non-violence and truthfulness.
• Many scholars believe that this Krishna was the same as the Krishna of later tradition, though this is debated.

About the Chandogya Upanishad:

• It belongs to the Samaveda.
• It is one of the oldest and longest Upanishads, in prose.
• It has eight chapters.
• It contains the famous teaching of Uddalaka Aruni to his son Shvetaketu, with the "Mahavakya" "Tat tvam asi" ("That thou art"), which means that the individual self is identical with the ultimate reality.
• It contains the story of Satyakama Jabala, a boy who did not know who his father was, but was accepted as a student because he told the truth.
• It explains the meaning of the syllable "Om" and the importance of chanting.

Development of the Krishna tradition:

• Krishna appears as a hero and teacher in the Mahabharata and the Bhagavad Gita.
• The "Bhagavata" cult (worship of Vasudeva-Krishna) developed by the 2nd century BCE.
• The Heliodorus pillar at Besnagar (Vidisha), set up by a Greek ambassador, refers to "Vasudeva, the god of gods".
• Later, the Harivamsha and the Puranas (especially the Bhagavata Purana) describe Krishna's childhood in Vrindavan and his life in Mathura and Dwarka.

Why not the other options?

• ✗The Rigveda does not mention Krishna, son of Devaki.
• ✗The Brihadaranyaka and Katha Upanishads do not contain this reference.