score:7
The earliest reference I can find is Justin Martyr (c.β100 β 165 AD) who wrote in Dialog With Trypho:
But when the Child was born in Bethlehem, since Joseph could not find a lodging in that village, he took up his quarters in a certain cave near the village; and while they were there Mary brought forth the Christ and placed Him in a manger. (1)
Upvote:3
There is a short prequel to the gospels called the Protevangelium of James, supposedly written about 150 ad., but it is noncanonical and certainly not an eyewitness account. The author claims to have been walking in a field and this vision just came to him. Although I don't believe that it is authentic history, it does mention Mary going into a cave with her midwife, and there is a big flash of light when Jesus is born. Interesting, but nothing more.