Tawḥīd (Arabic: التَوحيد), or monotheism is the most fundamental doctrine in Islam, which means believing in the oneness and uniqueness of God and not ascribing any partners to Him. The message of the Prophet Muhammad (s) since the beginning of his mission was based on the call for tawhid and the rejection of polytheism. Tawhid is a major theme in the Qur'an and hadiths. Al-Tawhid is also one of the names of sura 112 of the Qur'an, which presents a brief but deep expression of the doctrine.
In Islam, tawhid stands in contrast to shirk (ascribing partners to God). Muslim theologians have counted a number of levels for tawhid: al-tawhid al-dhati (unity of the divine essence), which refers to the belief in God as the only God and negating any partners for Him; al-tawhid al-sifati (unity of the divine attributes), which refers to the identity of divine essence with divine attributes; al-tawhid al-af'ali (unity of actions), which refers to the fact that God is the only independent cause and that He needs no assistance for His actions; al-tawhid al-'ibadi (unity of worship), which refers to the fact that no one other than God deserves to be worshiped.
There are various arguments for tawhid in the Qur'an, hadiths, and the works of Muslim philosophers and theologians. The argument of tamanu' (Argument of Antagonism), the argument from appointing prophets, and the Argument of the Impossibility of Multiplicity are some of the arguments presented by Muslim theologians and philosophers for tawhid.
Some Sunnis, including Ibn Taymiyya, Muhammad b. 'Abd al-Wahhab, and 'Abd al-'Aziz Bin Baz, have regarded asking for shafa'a and tawassul as instances of shirk. Based on the teachings of the Qur'an, other Muslims reject this idea and maintain that there is no problem in asking for intercession when the intercessor is not considered divine and asking for intercession is done just to honor the intercessor and get closer to God through them.
Shiite scholars have written extensively on the topic of tawhid. Some of their works are devoted exclusively to tawhid, and others are parts of works on theology. Some of these books are al-Tawhid by al-Shaykh al-Saduq, Gowhar-i murad by 'Abd al-Razzaq Lahiji,