"...If the party liable is mentally deficient, or weak, or unable Himself to dictate, Let his guardian dictate faithfully, and get two witnesses, out of your own men, and if there are not two men, then a man and two women, such as ye choose, for witnesses, so that if one of them errs, the other can remind her. The witnesses should not refuse when they are called on (For evidence)." - Quran 2:282 (read more)

"Narrated 'Abdullah bin 'Umar : The Jew brought to the Prophet a man and a woman from amongst them who have committed (adultery) illegal sexual intercourse. He ordered both of them to be stoned (to death), near the place of offering the funeral prayers beside the mosque." - Sahih Bukhari 7:62:64 (read more)

"Allah says: "There is no compulsion in religion"...But, this verse is abrogated by the verse of "fighting...Therefore, all people of the world should be called to Islam. If anyone of them refuses to do so, or refuses to pay the Jizya they should be fought till they are killed. This is the meaning of compulsion. In the Sahih, the Prophet said: "Allah wonders at those people who will enter Paradise in chains", meaning prisoners brought in chains to the Islamic state, then they embrace Islam..." - Ibn Kathir