I was wondering what is meant by At-Tirmidhi when he calls a hadith Hasan or Hasan-Ghareeb? Does it mean that the hadith is sound? I heard someone say that it means the hadith is weak.

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

Imaam At-Tirmithi classifies the hadeeth as Hasan (good) when it has slight weakness (in its chain of narration) and was reported through another (stronger) chain of narration. This slight weakness could mean one of the following things: the chain of narration includes a reporter classified by Hadeeth scholars as either Mastoor (whose inward trustworthiness cannot be affirmed by scholars), or weak due to his poor memory or due to making mistakes in his narrations, or known for Ikhtilaat (weak or loss of memory in old age); or it could be a narrator who practices Tadlees (who narrates a hadeeth which he did not hear directly from his Shaykh, without mentioning the name of the third party from whom he heard it) when he uses the ʻAn'anah (saying: "On the authority of so-and-so, from so-and-so"); or it could be the hadeeth which has a slightly-interrupted chain of narration.

Al-Haafith Ibn Hajar wrote:

"According to At-Tirmithi, all these are kinds of Hasan ahaadeeth provided that three conditions are met:

1) There is no narrator accused of lying in the Isnaad (chain of narration).

2) The chain of narration is not Shaath (i.e. a hadeeth narrated by a trustworthy narrator but it differs from the narration of other trustworthy narrators).

3) More than one narration of the same meaning is reported through one or more other chain of narration, and they are of different levels of strength, reinforcing one another." [An-Nukat ʻala Muqaddimat Ibn As-Salaah]

Ibn As-Salaah wrote, "It was narrated to us from Abu 'Eesa At-Tirmithi that by a Hasan hadeeth, he means  a hadeeth that is not Shaath, nor contains a reporter accused of lying in the Isnaad, and which is reported through more than one chain of narration." [Maʻrifat ʻUloom Al-Hadeeth]

It should be noted that the Hasan hadeeth according to At-Tirmithi is one category of the Hasan ahaadeeth in general. There are two categories of Hasan ahaadeeth: Hasan Lighayrihi (good due to other ahaadeeth, i.e. it is not Hasan in itself but due to other ahaadeeth reinforcing it) and Hasan Lithaatihi (Hasan in and of itself).

Ibn As-Salaah wrote:

"I have carefully investigated their (scholars of Hadeeth) methodology and deduced that the Hasan hadeeth is of two categories:

1) One with an Isnaad containing a reporter who is Mastoor but is not totally careless in his narration provided that a similar text is reported through another Isnaad as well. It means that the chain of narration must not include a narrator who was accused of deliberate lying in his narrations, nor is there any other reason entailing declaring him impious (and accordingly discrediting his narrations). Moreover, the Matn (text) of the hadeeth must be reported through one different chain of narration or more, reinforcing one another, or similar corroborating narrations of the same meaning. This excludes the Shaath and Munkar (when a narration which contradicts another authentic hadeeth is reported by a weak narrator). The relevant statement of At-Tirmithi applies to this category.

2) One with an Isnaad containing a reporter who is known to be truthful and reliable but did not reach the degree of the reporters approved by Al-Bukhaari in his Saheeh because he is less than them in memorization and precision. However, he is more trustworthy compared to the reporters whose narrations are declared Munkar if narrated solely on their authority. And in addition to not being Shaath or Munkar, the report must be free of ʻIllah (hidden defects). The statement of Al-Khattaabi applies to the second category."

As for the Ghareeb hadeeth, it is the hadeeth or version reported by one reliable or unreliable narrator which differs in context with another hadeeth or version reported by a group of reliable narrators, either pertaining to the text or the chain of narration. If a hadeeth is classified as Hasan-Ghareeb, it means that the conditions of the Hasan hadeeth apply to it, yet some of the reporters individually narrated something different pertaining to its text or chain of narration. Imaam At-Tirmithi uses the classification Hasan-Ghareeb frequently in his Hadeeth collection.

One of the examples of the Hasan hadeeth reported in Sunan At-Tirmithi is the following, "Muhammad ibn Ismaaʻeel narrated on the authority of Maalik ibn Ismaaʻeel from Israa'eel ibn Yoonus from Yoosuf ibn Abi Burdah from his father that Aa’ishah said, 'The Prophet, sallallaahu ‘alayhi wa sallam, used to say upon exiting the toilet, ‘Ghufraanak (I seek Your forgiveness).’' At-Tirmithi commented on this hadeeth, saying, 'This is a Hasan-Ghareeb hadeeth that we do not know of except on the authority of Israa’eel from Yoosuf ibn Abi Burdah.'"

Allah knows best.