What are the ahaadeeth that may be used as shar’i evidence?

The ahaadeeth which must be accepted and taken as evidence
are the acceptable ahaadeeth: those which are saheeh or hasan. As for da’eef
(weak) and false ahaadeeth, it is not permissible to use them as evidence
with regard to shar’i rulings. 

Al-Imam al-Shaafa’i said in al-Risaalah (p. 463): 

It is obligatory to accept a report once it is proven, even
if the imams did not act upon this report. End quote. In other words: once a
hadeeth is proven to be from the Prophet (peace and blessings of Allaah
be upon him), it is obligatory to act upon it. 

‘Abd-Allaah ibn al-Imam Ahmad ibn Hanbal said: 

I asked my father about a man who has books which contain the
words of the Messenger of Allaah (peace and blessings of Allaah be upon
him) and the Sahaabah and Taabi’een, but the man does not know how to
recognize da’eef (weak) and matrook (rejected) ahaadeeth, and he cannot tell
a strong isnaad from a weak one. Is it permissible for him to act upon
whatever he wants and to choose reports and issue fatwas and act on that
basis? 

He said: he should not do anything until he finds out which
ahaadeeth are sound and may be acted upon, so that he will be acting upon a
sound basis. He should ask the scholars about that. End quote. 

I’laam al-Muwaqqi’een (4/179). 

Imam Muslim (may Allaah have mercy on him) said: 

You should note, may Allaah have mercy on you, that knowledge
of hadeeth and determining which are sound and which are unsound is the role
of the scholars of hadeeth only, because they are the ones who have
memorized the people’s reports and they know them well, unlike others. So
the basic principle on which they base their beliefs is the Sunnahs and
reports which were transmitted from one era to the next from our Prophet
(peace and blessings of Allaah be upon him) until the present day. End
quote. 

Al-Tamyeez, p. 218 

Imam al-Sarkhasi (may Allaah have mercy on him) said: 

Not following a saheeh hadeeth from the Messenger of Allaah
(peace and blessings of Allaah be upon him) is haraam, just as doing
what is contrary to it is haraam. End quote. 

Usool al-Sarkhasi (2/7). 

Al-Nawawi (may Allaah have mercy on him) said: 

The scholars said: Ahaadeeth are of three types: saheeh
(sound), hasan (good) and da’eef (weak). They said: It is only permissible
to quote as evidence concerning rulings those ahaadeeth which are saheeh or
hasan. As for those which are da’eef, it is not permissible to quote them as
evidence with regard to rulings or beliefs, but it is permissible to narrate
them and act upon them with regard to matters other than rulings, such as
stories, virtuous deeds, and offering encouragements and warnings. End
quote. 

Al-Majmoo’ (1/98). 

Al-Haafiz Ibn Rajab (may Allaah have mercy on him) said: 

With regard to the imams and fuqaha’ of ahl al-hadeeth, they
follow the saheeh hadeeth wherever it is. End quote. 

Fadl ‘Ilm al-Hadeeth (p. 57). 

Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him)
said: 

It is not permissible to rely in matters of sharee’ah on
da’eef ahaadeeth which are not saheeh or hasan. End quote. 

Majmoo’ al-Fataawa (1/250). 

Al-‘Allaamah Zakariya al-Ansaari (may Allaah have mercy on
him) said: 

The one who wants to quote as evidence a hadeeth from the
Sunans or Musnads, if he is qualified to distinguish between what may be
quoted as evidence and what may not, then he should not quote it as evidence
unless he examines its isnaad and its narrators. Otherwise if he can find
one of the imams who classed it as saheeh or hasan, he may follow him,
otherwise he should not quote it as evidence. End quote. 

Fath al-Baaqi Sharh Alfiyyat al-‘Iraaqi 

See also the answer to question no.
115125 for information on the
ruling on one who rejects a saheeh hadeeth and no.
79163 for information on the conditions of a saheeh hadeeth. 

And Allaah knows best.