The Sabi'un or Sabians
There is a difference of opinion over the identity of the Sabians. Sufyan Ath-Thawri said that Layth bin Abu Sulaym said that Mujahid said that, "The Sabians are between the Majus, the Jews and the Christians. They do not have a specific religion.'' Similar is reported from Ibn Abi Najih. Similar statements were attributed to `Ata' and Sa`id bin Jubayr. They (others) say that the Sabians are a sect among the People of the Book who used to read the Zabur (Psalms), others say that they are a people who worshipped the angels or the stars. It appears that the closest opinion to the truth, and Allah knows best, is Mujahid's statement and those who agree with him like Wahb bin Munabbih, that the Sabians are neither Jews nor Christians nor Majus nor polytheists. Rather, they did not have a specific religion that they followed and enforced, because they remained living according to their Fitrah (instinctual nature). This is why the idolators used to call whoever embraced Islam a `Sabi', meaning, that he abandoned all religions that existed on the earth. Some scholars stated that the Sabians are those who never received a message by any Prophet. And Allah knows best.
[وَإِذْ أَخَذْنَا مِيثَـقَكُمْ وَرَفَعْنَا فَوْقَكُمُ الطُّورَ خُذُواْ مَآ ءَاتَيْنَـكُم بِقُوَّةٍ وَاذْكُرُواْ مَا فِيهِ لَعَلَّكُمْ تَتَّقُونَ - ثُمَّ تَوَلَّيْتُم مِّن بَعْدِ ذلِكَ فَلَوْلاَ فَضْلُ اللَّهِ عَلَيْكُمْ وَرَحْمَتُهُ لَكُنتُم مِّنَ الْخَـسِرِينَ ]
(63. And (O Children of Isra'il, remember) when We took your covenant and We raised above you the Mount (saying): "Hold fast to that which We have given you, and remember that which is therein so that you may acquire Taqwa.) (64. Then after that you turned away. Had it not been for the grace and mercy of Allah upon you, indeed you would have been among the losers.)