Imaam al-Dhahabi (may Allaah be pleased with him) said in his biography of al-Murtada ‘Ali ibn Husayn ibn Moosa al-Moosawi (d. 436 AH): I said, he was the compiler of the book Nahj al-Balaaghah which is attributed to Imam ‘Ali (may Allaah be pleased with him), but the reports contained therein have are no isnaads. Some of it is false and some of it is true, but it contains some frabricated reports of things that the Imam would never have said. But who is the fair-minded man who would look at it in an objective manner?! It was said that it was compiled by his brother Shareef al-Radiy. It includes slander against the companions of the Messenger of Allaah (peace and blessings of Allaah be upon him); we seek refuge with Allaah from knowledge that is of no benefit.
Siyar A’laam al-Nubala’, 17/589
Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him) said: “Most of the khutab (sermons) that the author of Nahj al-Balaaghah includes in his book are lies against ‘Ali. ‘Ali (may Allaah be pleased with him) is too noble and too worthy to have uttered such words. But these people fabricated lies and thought that they were praise, but they are neither truth nor praise. Whoever says that the words of ‘Ali or any other human being are above the words of any other created being is mistaken, for the words of the Prophet (peace and blessings of Allaah be upon him) are above his words, and both of them are created beings. Moreover the correct meanings that are to be found in the words of ‘Ali are to be found in the words of others, but the author of Nahj al-Balaaghah and his ilk took many of the things that people say and made them the words of ‘Ali. There are some words narrated from ‘Ali that he did say, and some of them are true words that would have been befitting for him to say, but in fact they are the words of others. Hence in Kalaam al-Bayaan wa’l-Tabyeen by al-Haafiz and in other books there are words narrated from people other than ‘Ali and the author of Nahj al-Balaaghah attributed them to ‘Ali. If these sermons which were transmitted in Nahj al-Balaaghah were really spoken by ‘Ali, they would have been found in other books that existed before this book was written, and they would have been narrated from ‘Ali with isnaads and otherwise. It is known from those who are well versed in the study of narrations that many of them (these sermons) – indeed most of them – were unknown before this, therefore it may be concluded that they are fabrications. So the narrator should state in which book they are mentioned, who narrated it from ‘Ali, and what its isnaad is. Otherwise, anybody could say something and claim that it was said by ‘Ali. Those who are well-versed in the knowledge of the hadeeth scholars and of reports and isnaads and are able to tell what is sound and what is not sound would know that these people who transmitted reports from ‘Ali are the least likely people to know about reports and be able to distinguish the sound from the unsound.
Manhaaj al-Sunnah al-Nabawiyyah, 8/55.