I have often been told that when I quote a hadith I need to make sure it is authentic. For this reason I have read several Islamic books on how to authenticate a hadith. The basic summary is:
1. It must have a complete chain.
2. The narrators in the chain must be reliable.
3. The chain must be historically possible.
But I have come across another important criteria which I now have references for.
g) Traditions containing such remarks of the Prophet as may not be a part of his prophetic vocation, or such expressions as are clearly unsuitable for him, should be rejected. (Muhammad Zubayr Siddiqi, Hadith Literature - Its Origin, Development & Special Features, Cambridge: U.K., The Islamic Text Society, 2008, p. 114)
God willing I shall begin this book with Isma'il son of Ibrahim and mention those of his offspring who were the ancestors of God's apostle one by one with what is known about them, taking no account of Isma'il's other children, for the sake of brevity, confining myself to the prophet's biography and omitting some of the things which Ibn Ishaq has recorded in this book in which there is no mention of the apostle and about which the Quran says nothing and which are not relevant to anything in this book or an explanation of it or evidence for it; poems which he quotes that no authority on poetry whom I have met knows of; things which it is disgraceful to discuss; matters which would distress certain people; and such reports as al-Bakka'i told me he could not accept as trustworthy—all these things I have omitted. But God willing I shall give a full account of everything else so far as it is known and trustworthy tradition is available. (“Ibn Hisham’s Notes”, translated by A. Guillaume in Ibn Ishaq, Sirat Rasul Allah, translated as, The Life of Muhammad, Karachi: Oxford University Press, 1998, p. 691)
It seems that for a hadith to be authentic it must also honour Muhammad.
This is a very poor method for doing history because it limits our knowledge of this person to some predetermined standard rather than to the evidence that is available.
Non-Muslim historians actually hold that an embarrassing account from a sympathetic source is more likely to be true because there is no motive to dishonour the person. Therefore, the criteria of embarrassment, rather than providing grounds for rejecting a hadith, actually provide grounds for accepting a hadith.
Ibn Hisham clearly explains how he has edited Ibn Ishaq's sirat and it includes removing "things which it is disgraceful to discuss; matters which would distress certain people." Ibn Hisham's approach seems to be based on the Qur'an itself.
O you who believe! Ask not questions about things which, if made plain to you, may cause you trouble. ... (Qur’an 5:101, Yusuf Ali)
I think it would profit Christians and Muslims to discuss whether this is a valid historical method?
If you know about similar quotes to those I have given please share them as a comment.