Some claim that for a testimony to be accepted, it requires four witnesses. Therefore, if a Hadith attributed to the prophet has four different “authentic” isnads, it must be accepted.
The requirement for four witnesses comes from the Quran and is regarding adultery which can be seen in the following verses.
[4:15] Those who commit adultery among your women, you must have four witnesses against them, from among you. If they do bear witness, then you shall keep such women in their homes until they die, or until GOD creates an exit for them.
وَٱلَّـٰتِى يَأْتِينَ ٱلْفَـٰحِشَةَ مِن نِّسَآئِكُمْ فَٱسْتَشْهِدُوا۟ عَلَيْهِنَّ أَرْبَعَةً مِّنكُمْ فَإِن شَهِدُوا۟ فَأَمْسِكُوهُنَّ فِى ٱلْبُيُوتِ حَتَّىٰ يَتَوَفَّىٰهُنَّ ٱلْمَوْتُ أَوْ يَجْعَلَ ٱللَّهُ لَهُنَّ سَبِيلًا
[24:4] Those who accuse married women of adultery, then fail to produce four witnesses, you shall whip them eighty lashes, and do not accept any testimony from them; they are wicked.
وَٱلَّذِينَ يَرْمُونَ ٱلْمُحْصَنَـٰتِ ثُمَّ لَمْ يَأْتُوا۟ بِأَرْبَعَةِ شُهَدَآءَ فَٱجْلِدُوهُمْ ثَمَـٰنِينَ جَلْدَةً وَلَا تَقْبَلُوا۟ لَهُمْ شَهَـٰدَةً أَبَدًا وَأُو۟لَـٰٓئِكَ هُمُ ٱلْفَـٰسِقُونَ
[24:13] Only if they produced four witnesses (you may believe them). If they fail to produce the witnesses, then they are, according to GOD, liars.
لَّوْلَا جَآءُو عَلَيْهِ بِأَرْبَعَةِ شُهَدَآءَ فَإِذْ لَمْ يَأْتُوا۟ بِٱلشُّهَدَآءِ فَأُو۟لَـٰٓئِكَ عِندَ ٱللَّهِ هُمُ ٱلْكَـٰذِبُونَ
The reason why this logic does not work for Hadith is that the concept of four witnesses only works for the first-hand witnesses to the account, but Hadith is based on a chain of narrators (isnad) spanning typically about 7 links from the prophet to the compiler.
So, for instance, assuming the original four witnesses were truthful, did not misunderstand what they witnessed, and did not conspire, then their testimony can be accepted. But what if we don’t hear the testimony directly from the four witnesses but from a secondary source, then there will need to be four secondary witnesses for each of the original witnesses who can all corroborate the testimonies of the original witnesses.
Therefore, if we accept secondary testimonies, we would need 16 testimonies who heard it from the original four witnesses. This shows that for n number of individuals in a respective isnad, we will need 4n witnesses for each link in the chain. Such that if there are 7 links in the chain, the compiler would need 47 or 16,384 witnesses if we want to accept this testimony as authentic.
Even the most mass-transmitted Hadith does not even come close to obtaining this many witnesses by the time of compilation. For example, scholars attempted to compile as many isnads as they could find for the below Hadith that they considered mass transmitted but were not even close to the requirement that would be required if we apply the four witness provision set by the Quran.
Abu Huraira reported the Prophet saying: (that Almighty Allah has said) Every act of the son of Adam is for him; every good deed will receive tenfold except fasting. It is [exclusively] meant for me, and I [alone] will reward it. He abandons his food for My sake and abandons drinking for My sake and abandons his pleasure for My sake. When any one of you is fasting he should neither indulge in sex nor use obscene language. If anyone reviles him he should say, “I am fasting.” The one who fasts has two [occasions] of joy: one when he breaks the fast and one on the day when he will meet his Lord. And the breath [of a fasting person] is sweeter to Allah than the fragrance of musk
