Sheikh Salman al-Oadah
Single-narrator hadîth are a topic about which people of knowledge have disagreed. The majority of scholars are of the opinion that they provide a strong assumption (zann) of their authenticity unless they are supported by other indicators of authenticity.
Single-narrator hadîth can be strengthened to the point where their authenticity is certain and indisputable. This can be brought about by any number of supportive indicators. For instance, the report can come to us by way of more than one completely different chain of transmission and be transmitted from a narrator with many different chains of transmission. Its meaning may be supported by the Qur’ân, the Sunnah, or the unquestionable principles of the faith. The report may enjoy the unanimous acceptance of the Muslim Ummah.
The strong assumption that is provided by single-narrator hadîth is not the same as the conjecture that the Qur’ân condemns the unbelievers for following. Strong assumption is recognized by all scholars of Islamic Law, and is acted upon in numerous circumstances. If people were required to have absolute knowledge in every situation, life would become unbearably difficult if not utterly impossible. Imagine, for instance, demanding absolute certainty in questions of purification, or in our acts of worship, or in our commercial transactions and civil contracts. A preponderance of belief that is based upon careful consideration, and not upon personal whims, has the same legal recognition as certainty. This has nothing to do with the speculations and false conjectures of the unbelievers.
Can we say that the testimony in court of four witnesses who are deemed to be reliable of good character provides us with absolute certainty of the veracity of that testimony? Is it at all possible that those witnesses have colluded in offering false testimony against an innocent person? However, such evidence is upheld in a court of law.
From this, we know that single-narrator hadîth fall into two categories:
1- Single-narrator hadîth supported by other indicators of authenticity that strengthen them to the point that we are certain of their authenticity.
2- Single-narrator hadîth that are not supported by other indicators and that provide us with a strong assumption of their authenticity.
And Allah knows best.
Source: Islam Today