As far as I'm aware (and I'm an archaeologist not a historian) the Asokan edicts are the first textual records of Siddhartha Gautama Buddha. The earliest Buddhist texts appear to primarily date from around the 1st century CE - although they are all theoretically much older teachings/histories that were first written down in the 1st century CE.

edit: Archaeologically speaking, recent excavations at Lumbini (identified by Asoka as the birthplace of Buddha) suggested the earliest phase of formal worship at the site dated to around the middle of the 6th century BC. It's difficult to conclusively say this original tree-shrine was a Buddhist shrine, although the Asokan period temple effectively copied the structural form, and the Asokan period monastic structures around the central shrine also copied timber structures from the 6th century. However, if we accept the initial tree-shrine as Buddhist, then you're looking at the Buddha being born somewhere around the late 7th century early 6th century BCE. National Geographic article on Lumbini excavations

Peer-reviewed Antiquity article on excavations

As Gautama was supposedly born into a royal family (subsumed into Kosala soon after), wouldn't there be some degree of records around the royal family? It seems like the vast majority of pre-modern writing involves documenting the royal and noble classes, no matter the culture and region.