Dr. Salah Sultan, President of the American Center for Islamic Research, Columbus, Ohio, and Member of the European Council for Fatwa and Research, states: “It is better to avoid these kinds of soap as the market is full of many other varieties that are fat-free. Prophet Muhammad (peace and blessings be upon him) is reported to have said: “Leave what makes you uncertain in favor of what makes you certain.”
however, imam ibn Hazm is of the view that even a non-zabihah beef can be used in producing such substances, as he argues that the verse in the Qur’an only specifies the prohibition of pig to its meat (Forbidden to you is the flesh of swine…) (Al-Mai’dah: 5), and therefore, it is applicable to any similar thing, such as the non-zabiha beef, as long as the meat is not used. The basis for this argument is the hadith reported on the authority of Maymona (may Allah be pleased with him) advising her to make use of the leather of the dead animal, saying: “Would you take its leather and make use of it?”
therefore, I cannot say it is haram to use such soap though it is highly recommended to avoid it.”