In the course of evaluating a Hadeeth's authenticity, scholars of Hadeeth have to investigate each person in its chain(s) of narrators for two things:
The notable exception to this rule is the Sahaba, the first generation of Muslims who were the companions of Prophet Muhammad ﷺ. They are all considered to be trustworthy.
Is there a reason why are they exempted from this check?
The Sahaba (who comprise of the Muhajirun and Ansar) are deemed trustworthy for many reasons:
Allah attests to their truthfulness and success
لِلْفُقَرَاء الْمُهَاجِرِينَ الَّذِينَ أُخْرِجُوا مِن دِيارِهِمْ وَأَمْوَالِهِمْ يَبْتَغُونَ فَضْلاً مِّنَ اللَّهِ وَرِضْوَاناً وَيَنصُرُونَ اللَّهَ وَرَسُولَهُ أُوْلَئِكَ هُمُ الصَّادِقُونَ . وَالَّذِينَ تَبَوَّؤُوا الدَّارَ وَالْإِيمَانَ مِن قَبْلِهِمْ يُحِبُّونَ مَنْ هَاجَرَ إِلَيْهِمْ وَلَا يَجِدُونَ فِي صُدُورِهِمْ حَاجَةً مِّمَّا أُوتُوا وَيُؤْثِرُونَ عَلَى أَنفُسِهِمْ وَلَوْ كَانَ بِهِمْ خَصَاصَةٌ وَمَن يُوقَ شُحَّ نَفْسِهِ فَأُوْلَئِكَ هُمُ الْمُفْلِحُونَ
(And there is also a share in this booty) for the poor emigrants, who were expelled from their homes and their property, seeking Bounties from Allah and to please Him. And helping Allah (i.e. helping His religion) and His Messenger (Muhammad ﷺ). Such are indeed the truthful (to what they say); And those who, before them, had homes (in Al-Madinah) and had adopted the Faith, love those who emigrate to them, and have no jealousy in their breasts for that which they have been given (from the booty of Bani An-Nadir), and give them (emigrants) preference over themselves, even though they were in need of that. And whosoever is saved from his own covetousness, such are they who will be the successful.
Al-Hashr:8-9
So Allah exonerates them from the possibility of deceiving for the sake of personal gain.
Allah is pleased with them
وَالسَّابِقُونَ الأَوَّلُونَ مِنَ الْمُهَاجِرِينَ وَالأَنصَارِ وَالَّذِينَ اتَّبَعُوهُم بِإِحْسَانٍ رَّضِيَ اللّهُ عَنْهُمْ وَرَضُواْ عَنْهُ وَأَعَدَّ لَهُمْ جَنَّاتٍ تَجْرِي تَحْتَهَا الأَنْهَارُ خَالِدِينَ فِيهَا أَبَداً ذَلِكَ الْفَوْزُ الْعَظِيمُ
And the first to embrace Islam of the Muhajirun (those who migrated from Makkah to Al-Madinah) and the Ansar (the citizens of Al-Madinah who helped and gave aid to the Muhajirun) and also those who followed them exactly (in Faith). Allah is well-pleased with them as they are well-pleased with Him. He has prepared for them Gardens under which rivers flow (Paradise), to dwell therein forever. That is the supreme success.
at-Tawbah:100
This verse qualifies the Muhajirun, Ansar and whoever follows in their footsteps from later generations as those who Allah is pleased with. Allah's being pleased with the later generations is conditional to their following the example in faith set by the Sahaba, while Allah's pleasure with the Sahaba is unconditional.
لَقَدْ رَضِيَ اللَّهُ عَنِ الْمُؤْمِنِينَ إِذْ يُبَايِعُونَكَ تَحْتَ الشَّجَرَةِ فَعَلِمَ مَا فِي قُلُوبِهِمْ فَأَنزَلَ السَّكِينَةَ عَلَيْهِمْ وَأَثَابَهُمْ فَتْحاً قَرِيباً
Indeed, Allah was pleased with the believers when they gave their pledge to you (O Muhammad ﷺ) under the tree, He knew what was in their hearts, and He sent down calmness and tranquillity upon them, and He rewarded them with a near victory
Al-Fath:18
