The earliest likely mention is in the Vishnu Parva of the Harivamsha, in Markandeya's discourse, where Vishnu and Shiva are declared one. The composite image that follows shows the two gods sharing a single body, split vertically, Shiva's ascetic half on one side and Vishnu's royal half on the other. Brahma has no place in it, which makes Harihara the two-god answer to the three-god trinity.
Carved Harihara images survive at the Badami caves from the sixth century and at the thirteenth-century Harihareshwara temple, with further examples in Tamil Nadu, Rajasthan, Nepal, Cambodia and Indonesia. The Kailasa temple at Ellora has a Harihara shrine and a painted Brahma but no three-god programme.
The same form as carved image rather than text; the geographical spread shows it was a pan-regional idea.
Sources: Harihara — Wikipedia
Vishnu and Shiva merge into a single form — to end a dispute among devotees, to kill Guhasura whom neither could kill alone, or at Parvati's prayer to reconcile two serpent kings. Shiva is the right half, Vishnu the left, in every telling.
Genuine three-god scenes carved on Ellora's Kailasa: Lingodbhava with Brahma and Vishnu searching the pillar, Tripurantaka with Brahma as charioteer and Vishnu as arrow, and Harihara with Nandi and Garuda.
A Khmer hilltop temple of Yasovarman I with three towers for Shiva, Vishnu and Brahma — a rare true Trimurti set — within a Cambodian tradition whose main composite cult was Harihara.
The Maitri Upanishad names Brahma, Rudra and Vishnu together as portions of one being, matched to the three gunas.
The sage Atri and his wife Anasuya earn great merit through their conduct and austerity.
A seven-metre three-faced bust fills the south wall of Elephanta's Cave 1.
Three Pallava shrines on one platform: Shiva in the centre, Vishnu on the right, and a disputed left shrine read as Brahma by Vogel but as Subrahmanya or Brahmasasta by later scholars; a Durga panel stands beside.
During the dissolution Vishnu lies on the serpent Ananta in the cosmic waters.