This one is from sunan abu dawuud but it is still considered authentic by many if not the majority of scholars.
"A blind man had a female slave who had borne him a child (umm walad) who reviled the Prophet (peace and blessings of Allaah be upon him) and disparaged him, and he told her not to do that but she did not stop, and he rebuked her but she paid no heed.
One night she started to disparage and revile the Prophet (peace and blessings of Allaah be upon him), so he took a dagger and put it in her stomach and pressed on it and killed her. There fell between her legs a child who was smeared with the blood that was there.
The next morning mention of that was made to the Prophet (peace and blessings of Allaah be upon him) and he assembled the people and said: “I adjure by Allaah the man who did this to stand up.”
The blind man stood up and came through the people, trembling, and he came and sat before the Prophet (peace and blessings of Allaah be upon him). He said: O Messenger of Allaah, I am the one who did it. She used to revile you and disparage you, and I told her not to do it but she did not stop, and I rebuked her but she paid no heed. I have two sons from her who are like two pearls, and she was good to me. Last night she started to revile you and disparage you, and I took a dagger and placed it on her stomach and I pressed on it until I killed her.
The Prophet (peace and blessings of Allaah be upon him) said: “Bear witness that no retaliation is due for her blood.” "
Number 4361.
There is just so many things that I feel is wrong with this hadith. I remember hearing this hadith for the first time from the radio from a sheikh and then the sheikh concluded that the ruling for blasphemy is death penalty, EVEN IF THE PERSON REPENTED.