I have made a few blog posts where I allude to my belief that the Sabians in the Koran were either the Himyarite Jews and/or other Jews and Christians of Yemen where Sheba/Saba was. That Sabians equals Sabeans.
Like my theory about the Ebonite origins of Islam.
The Koran Says Israel belongs to the Israelites.
And my theory that the Magi were from Yemen rather then Persia.
Only the first of those three is it even close to being relevant to the main point.
I am well aware that the most mainstream view is that they were the Mandeans, a Gnostic sect most famous for claiming to be followers of John The Baptist but not Jesus.
Thing is, that theory makes no sense to me. It seems to derive from them calling themselves a vaguely similar name.
But the Sabians of the Koran are considered "People of The Book". And that Book refers to one of or all of three specific parts of our Bible, The Torah, The Psalms and a Gospel. The Mandeans revere none of those books, as they reject Abraham, Moses, David and Jesus all as being False Prophets. The Prophets they have in common with traditional Judeo-Christian or Islamic line of Prophets ends with Noah or maybe Shem. They instead claim Aram was the first key post-Flood Prophet.
And I suspect they might very well deny The New Testament's claim that John The Baptist was an Israelite and instead claim he came from Aram, since his ministry was mostly East of The Jordan. He must have been in Perea to fall under Antipas' authority. Or maybe their John The Baptist was never meant to be the same as ours anyway, since theirs lived to see the Fall of Jerusalem in 70 AD.
As Gnostics, they weren't even truly Monotheistic. Which also rules them out as being People of The Book.
Muslim sources outside the Koran say they specifically followed the Zabur (The Psalms). That goes against my theory of them being the Sadducean Himyarites who were Torah only. But those sources are later and possibly based on misinformed assumptions. But it could also be the name comes from The Sabbath, and The Gospels tend to associate the Pharisees with The Sabbath more often then the Sadducees.