بسم الله الحمد لله والصلاة والسلام على رسول الله
Allaah سبحانه و تعالى Said in Al-Qur’an regarding those who conjecture about the gayb:
سَيَقُولُونَ ثَلَاثَةٌ رَّابِعُهُمْ كَلْبُهُمْ وَيَقُولُونَ خَمْسَةٌ سَادِسُهُمْ كَلْبُهُمْ رَجْمًا بِالْغَيْبِ ۖ وَيَقُولُونَ سَبْعَةٌ وَثَامِنُهُمْ كَلْبُهُمْ ۚ قُل رَّبِّي أَعْلَمُ بِعِدَّتِهِم مَّا يَعْلَمُهُمْ إِلَّا قَلِيلٌ ۗ فَلَا تُمَارِ فِيهِمْ إِلَّا مِرَاءً ظَاهِرًا وَلَا تَسْتَفْتِ فِيهِم مِّنْهُمْ أَحَدًا
“(Some) say they were three, the dog being the fourth among them; and (others) say they were five, the dog being the sixth – guessing at the unseen; (yet others) say they were seven, and the dog being the eighth. Say (O Muhammad صلى الله عليه وسلم): “My Lord knows best their number; none knows them but a few.” So debate not (about their number) except with the clear proof (which We have revealed to you). And consult not any of them (people of the Scripture – Jews and Christians) about (the affair of) the people of the Cave.” [Al-Kahf: 22]
Regarding the kutub of the People of the Book, Ibn Uthaymeen رحمه الله تعالى said:
“In summary, what has come in their books (i.e. the previously revealed books) – divides into two types and two things:
Firstly: That about which Allaah has narrated to us in the Qur’an, or the Messenger of Allaah صلى الله عليه وسلم (has narrated to us), then this is accepted, correct.
Secondly: What the People of the Book have narrated, then this does not escape from three situations:
i) The first situation: That our legislation testifies to its falsity, it is obligatory upon us to hold it as false and to reject it.
ii) The second situation: That whose truthfulness our legislation testifies to, so we accept it as true and accept it due to the witness of our legislation over it.
iii) The third situation: What is neither from this (the first) or that (the second), it is obligatory upon us to withhold, because they do not believe, and thus in their report, there will occur lying, change, addition and decrease.
[Source: ‘Sharh Riyaadh as-Saaliheen’, 1/450 (through “Mu’jam at-Ta’reefaat wad-Dawaabit wat-Taqseemaat wal-Fawaa’id Fil-Musannafaat al-I’itqaadiyyah lish-Shaykh Ibn al-Uthaymeen”, pp. 362-363)]
With regards to narration that originates from Judeo/Christian sources, Dr. Sadhan has a nice article as to their status, which can be found here
Allaah knows best.