How can disagreement be a mercy in matters such as the celebration of the Prophet's birthday (Mawlid) and the recitation of Surat Al-Kahf on Friday, where some consider them an innovation (bid'ah) and others consider them a Sunnah? And is dedicating the recitation of Surat Al-Kahf to Friday considered an innovation if the hadith regarding its virtue is not authentic?

The phrase "Disagreement among scholars is a mercy" applies to issues of ijtihad (independent reasoning) for which there is no specific textual evidence. As for disputes regarding Sunnah (Prophetic tradition) and bid'ah (innovation), evidence is required, for the basic principle regarding acts of worship is prohibition, while regarding customs it is permissibility. Any act whose cause existed during the time of the Prophet (peace be upon him) and the Salaf (early Muslims) yet they did not do it, then performing it after them is an innovation (bid'ah). Celebrating the Mawlid (Prophet's birthday) is an innovation because it was not done during the era of the Prophet (peace be upon him), the Rightly Guided Caliphs, the Companions, or the Tabi'in (successors of the Companions). None of the imams of the four schools of thought ever spoke of it. Rather, it was innovated by the Ubaydi (Fatimids) in imitation of the Christians. The Prophet (peace be upon him) said: "Whoever introduces into this matter of ours that which is not from it, it is rejected." And if a prohibitive evidence conflicts with a permissible one, the prohibitive one is given precedence. As for reciting Surat Al-Kahf on Friday, it is a Sunnah established by the Prophet's saying (peace be upon him): "Whoever recites Surat Al-Kahf on Friday, a light will illuminate for him what is between him and the Ancient House (Kaaba)," and in another narration: "what is between the two Fridays."