Warning against innovators – Shaikh Ibn Baaz رحمه الله
Imaam al-Bukhaaree رحمه الله said:
Book of Funerals: Chapter: Abusing the dead
Narrated `Aaishah رضي الله عنها: Allaah’s Apostle صلى الله عليه وسلم said: “Don’t abuse (revile) the dead, because they have reached the result of what they forwarded.”
[Saheeh al-Bukhaaree (1393, 6516)]
[Saheeh al-Bukhaaree (1394, 4770, 4801, 4971, 4972) and Saheeh Muslim (312)]
Shaikh Ibn Baaz رحمه الله said:
 If revilement is for the (purpose) of giving a warning, as in the (case) of an innovator so that he is not taken as an example to be followed, then there is no forbiddance in this; for it is (done) from the angle of warning and not revilement.
 By this the author (i.e. al-Bukhaaree) meant that the wicked ones were not included in the prohibition, and this is obvious from what has preceded, (this is done) to warn against them and defamation is not intended by it.
* Ibn al-Hajar al-`Asqalaanee رحمه الله said: “And the `Ulamaa’ have consensus on the permissibility of criticizing the unreliable narrators of (Hadeeth) whether they are alive or dead.” [Fathul Baaree Sharh Saheeh Al-Bukhaaree (3/259)]