Mutawaatir: That which has been narrated by a number of people in every level of the chain such that it is impossible for all of them to make a mistake or error. Ahad is the opposite in that it contains less than 4 narrators at any given level of the isnaad. Mutawatir has more than 4 at each level. (Please note that the exact number has been differed over, but this is what I have come across in my own personal study)
The people of kalam made a distinction in that they would only accept hadith mutawatir in matters of creed (aqeedah). The following are some proofs showing that this is a faulty position.
Allah (swt) says,
And it is not (proper) for the believers to go out to fight (Jihad) all together. Of every troop of them, a party (nafara) only should go forth, that they (who are left behind) may get instructions in (Islamic) religion, and that they may warn their people when they return to them, so that they may beware (of evil). [9:122]
Shafi’ee, Bukhari and many others said that nafara can refer to one person, or a small group.
Allah (swt) also said,
O you who believe! If a rebellious evil person comes to you with a news, verify it, lest you harm people in ignorance, and afterwards you become regretful to what you have done. [49:6]
So by this token, if a trustworthy person comes, then there is no need to verify.
The belief of not taking khabar al-ahaad led Amr bin Ubayd (a Mu’tazili) to say in regards to the hadith of qadr, “If ibn Mas’ood said it, I would call him a liar. If the Prophet(s) said it, I would reject it. If Allah said it, I would say that You did not create us for this” (a’udhubillah).
The Prophet(s) also normally sent just one person to teach people, for example Mus’ab ibn ‘Umayr to Medinah. They did not reject and ask for a mutawaatir group of people. Imam Shafi’ee also quoted the following hadith as a proof for khabarul waahid in his book ar-Risaala,
The Prophet(s) said, “May Allah brighten the face of a person who heard my speech, and then memorized it and understood it, and passed it on to others.” [Tirmidhi]
The action of the direction of the qibla changing from Jerusalem to Mecca is also a proof, as only one caller was sent out to the different masjids announcing it and everyone accepted the command.
The early scholars also had many statements regarding this issue.
People who deny khabar al-ahaad in matters of aqeedah argue that it does not constitute certain knowledge, and therefore cannot be acted upon. Certain knowldge and acting on knowledge are 2 different things that should not be confused. So for example, if a trustworthy person comes and says it is raining and to take an umbrella, you will act on it even though you might not be a full 100% sure of it. But you will still act on it since the person is trustworthy. If 10 people come into the room soaking wet and tell you its raining, then its ‘certain knowledge’. So whether or not khabar al-wahid gives certain knowledge or not is irrelevant in regards to acting on it.
Ibn Taymiyyah argued that khabar wahid can be certain knowledge depending on the situation, for example a distressed person saying that his father died. Also, he said that the hadith found in Bukhari and Muslim constitute certain knowledge since the ummah has agreed on them.
The most important point, perhaps, is that this differentiation was unkown to the Sahabah and early generations. It was made by the latter scholars. There is also no precise demarcation between what is mutawatir vs. ahad. According to one opinion, it is to have at least 10 people in each level. as-Suyuti tried to compile the hadith fitting this condition and only came up with ~110. This shows that making this distinction basically leaves us with no religion.
Lastly, the same people who use this distinction due so out of their principles of logic and philosophy and clearly reject mutawatir narrations when it goes against their ‘philosophy’. An example of this is the hadith of nuzool which will be covered later in great detail and is a mutawatir narration mentioned by 39 Sahabah.