Some puranas identify Buddha with Vishnu among them which is the oldest reference?
In puranas Buddha is mentioned one of the ten avatars of Lord Vishnu, usually the ninth avatar. The puranas that mention Buddha as the avatar of Vishnu are: Harivamsa (1.41) Vishnu Purana (3.18) Bhagavata Purana (1.3.24, 2.7.37, 11.4.23) Garuda Purana (1.1, 2.30.37, 3.15.26) Agni Purana (16) Naradiya Purana (2.72) Linga Purana (2.71) Padma Purana (3.252)
The Vishnu Purana mentions the sequential order from the oldest to the newest among the puranas
[Vishnu Purana - 3.6.20-23] 1. Brahma Purana 2. Padma Purana 3. Vishnu Purana 4. Shiva Purana 5. Bhagavata Purana 6. Narada Purana 7. Markandeya Purana 8. Agni Purana 9. Bhavisya Purana 10. Brahmavaivarta Purana 11. Linga Purana 12. Varaha Purana 13. Skanda Purana 14. Vamana Purana 15. Kurma Purana 16. Mastya Purana 17. Garuda Purana 18. Brahmanda Purana
According to Vishnu Puranas order Padma Purana is the oldest among Puranas which mention Buddha as Vishnu's avatar. Padma Purana 3.252