Ibn Sirin's quote
Of course the athar of ibn Sireen (Sirin) محمد بن سيرين which you may find quoted with a full narrator chain is widely known for a large audience (and maybe this is the source of Malik's quote). You'll find it for example in the introduction of sahih Muslim #25 or #26 in the 5th Chapter: "That Which is Related to the Statements ‘The Chain of Narration is from the Religion’; ‘Transmissions are not Taken Except from Trustworthy Narrators’; and ‘Criticism of the Narrators With What is Permissible Regarding Them, Even Obligatory and That It is not the Prohibited Kind of Backbiting, Rather it is the Defense of the Noble Sharī’ah’."
إِنَّ هَذَا الْعِلْمَ دِينٌ فَانْظُرُوا عَمَّنْ تَأْخُذُونَ دِينَكُمْ
‘Indeed this knowledge is faith, so carefully consider from whom you take your faith’.
"This knowledge is religion, so be careful from whom you take your religion." (source of this translation)
From this athar we can conclude that 'ilm ar-Rijal (the study of narrators and narrator chains) is very important. In this context the hadith about the best generations is also often quoted (see for example in sahih al-Bukhari and sahih Muslim). Some even tried to attribute this quote to the Prophet (), but there's no sane narrator chain which can support this claim as ibn Rajab said!
Note that even the brother of ibn Sirin (Anas ibn Sirin) has made the same statement and you may find it in some records of athar!
Note that the Arabic word: عِلم 'ilm is usually translated knowledge or science according the context!
Sources of Malik's quote
The quote of imam Malik can be found in most of his biographies and books on his virtues like tazien al-Mamalik bi Manqibi Malik of imam as-Suyuti and it is often quoted with a longer addition:
حَدَّثَنَا عَبْدُ الْوَارِثِ بْنُ سُفْيَانَ ، قَالَ : نا قَاسِمُ بْنُ أَصْبَغَ ، قَالَ : نا مُحَمَّدُ بْنُ إِسْمَاعِيلَ التِّرْمِذِيُّ ، قَالَ : سَمِعْتُ ابْنَ أَبِي أُوَيْسٍ يَقُولُ : سَمِعْتُ خَالِي مَالِكَ بْنَ أَنَسٍ ، يَقُولُ : إِنَّ هَذَا الْعِلْمَ دِينٌ ، فَانْظُرُوا عَمَّنْ تَأْخُذُونَ دِينَكُمْ ، لَقَدْ أَدْرَكْتُ سَبْعِينَ مِمَّنْ يَقُولُ : قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ، عِنْدَ هَذِهِ الأَسَاطِينِ ، وَأَشَارَ إِلَى مَسْجِدِ رَسُولِ اللَّهِ صلى الله عليه وسلم ، فَمَا أَخَذْتُ عَنْهُمْ شَيْئًا ، وَإِنَّ أَحَدَهُمْ لَوِ اؤْتُمِنَ عَلَى بَيْتِ مَالٍ لَكَانَ بِهِ أَمِينًا ، إِلا أَنَّهُمْ لَمْ يَكُونُوا مِنْ أَهْلِ الشَّأْنِ ، وَقَدِمَ عَلَيْنَا ابْنُ شِهَابٍ فَكُنَّا نَزْدَحِمُ عَلَى بَابِهِ.
We were told by abd al-Warith ibn Sufyan, who said, (we were told by) Qassim ibn Asbagh, said (we were told by) Muhammad ibn Isma'il at-Tirmidhi, said I've herad Ibn Uways saying, "I heard the husband of my (maternal) auntie Malik ibn Anas saying:
This knowledge is religion, so be careful from whom you take your religion.
I've witnessed (met) seventy among those who used to narrate saying such said the Messenger of Allah (Allahs prayers and blessing be upon him) said ... at these pillars" and he was pointing at the Mosque of the Messenger of Allah (Allahs prayers and blessing be upon him) (this means they were teaching at the mosque) "but I haven't taken from them anything (any hadith), but if any of them was employee at the bayt al-Mal (Public treasury) they would be trustworthy, but they were not people of this matter (knowledge).
Then ibn Shihab (az-Zuhri) came to Medina and we were trusting our self at his doors."
(Source ibn 'Abd al-Barr's book Al-Intiqâ' fî Fadâ'il al-Thalâthat al-A'immat al-Fuqahâ' Mâlik wal-Shâfi`î wa Abî Hanîfa p.16 الانتقاء في فضائل الثلاثة الأئمة الفقهاء لابن عبدالبر)
Ibn 'Abd-al-Barr added that he has connected chains saying something similar on the authority of Ashab ibn 'abd al-'Aziz أشهب بن عبد العزيز (the student of imam Malik and later mufti in Egypt), ibn Kinana 'Othman ابن كنانة عثمان (a student of Malik from Medina) and Bishr ibn 'Amr بشر بن عمر.
There's an other quote of Malik which I found somewhere were he advised a student saying:
اتق الله وانظر ممن تأخذ هذا الشأن أي العلم
Fear Allah and be careful from whom you take this matter (meaning the knowledge)