Salah behind a Mushrik

Q 1: What is the ruling on those who offer Salah behind grave worshippers? What is a Muslim's attitude toward a person who offers Salah behind the worshippers of graves even if this person does not commit acts of Shirk (associating others with Allah in His Divinity or worship) and does not know much about the books of monotheism? When this man went to ask about the books of Tawhid (Monotheism), a man said to him: These books are a double-edged tool?

A: Salah is invalid behind grave worshippers and a Muslim should advise his Muslim brother regarding the ruling on Salah behind grave worshippers. He should explain the ruling to him with proofs, may Allah guide him. Saying that (Part No. 7; Page No. 356) the books of Tawhid (Monotheism) are like a double-edged tool etc., is not correct because the books of Tawhid explain the obligation of worshipping Allah Alone, devotion and sincerity to Him and warn against Shirk (associating others with Allah in His Divinity and worship), its means and paths. These books also explain the pure way of following the Prophet (peace be upon him) depending on the Book of Allah, the sound Sunnah and the sayings of the Salaf (righteous predecessors) i.e. the Sahabah (Companions), the Tabi`un (Followers, the generation after the Companions of the Prophet) and those who followed them. These books are like the cure and light against the darkness of ignorance and Shirk because of what they contain of Qur'an and Hadith. There are many examples of these books such as, Tawhid Book by Ibn Khuzaymah, Sunnah Book by `Abdullah ibn Ahmad ibn Hanbal, and the reply of `Uthman ibn Sa`id Al-Darimy to Al-Jahmiyyah (a deviant Islamic sect denying some Attributes of Allah, claiming they are ascribed to people and cannot be ascribed to Allah). These books also include books of Shaykh-ul-Islam Ibn Taymiyyah, that of Ibn Al-Qayyim, books of Shaykh Muhammad ibn `Abdul-Wahhab and books of other scholars affiliated to Ahl Al-Sunnah wal-Jama`ah (those adhering to the Sunnah and the Muslim main body).May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.