It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that a man had an "umm walad" (a slave woman who had borne her master a child), from whom he had two sons like pearls. She used to insult the Prophet (peace and blessings be upon him); he would forbid her but she would not desist, and he would rebuke her but she paid no heed. Then one night she mentioned the Prophet (peace and blessings be upon him) (in an evil way), and the man could not bear it. He got up with a dagger, placed it on the woman's belly and pressed on it until he drove it through her. Then the Messenger of Allah (peace and blessings be upon him) said: "I bear witness that her blood is unavenged (no retribution is due for it)." This hadith is sahih (authentic) on the condition of Imam Muslim, but the two Shaykhs (Bukhari and Muslim) did not record it.