Eldad and Medad

The earliest known reference appears in the Shepherd of Hermas (an Apostolic Fathers writing and not one of the Apocrypha) that dates to roughly AD 150.

A lost work. Nicophoris of Constantinople (mid-AD 800s) wrote that it had 400 lines of text.

Background about the men "Eldad" and "Medad": Before Numbers 11:26-29, Moses took 70 elders to gather around the Tabernacle to witness the first gathering of quail. Eldad and Medad remained behind in these verses, yet the Spirit came on them, and they began to prophesy. Joshua found out and told Moses stop them, but Moses encouraged them and said he wished more people would prophesy.
Summary of this book: The contents are unknown, but it is quoted in the Shepherd of Hermas, a popular book amongst early Christians. Some scholars also believe it includes a line that James refers to in James 4:5: "Or do you think that Scripture says without reason that the Spirit he caused to live in us envies intensely"? The Scripture James refers to is not found in the OT but could align with Moses' wish that more would prophesy.