The religious movement of the Ibāḍiyya originated and was at first centred in Baṣra where its widely recognized spiritual leader Abū ʿUbayda Muslim b. Abī Karīma resided and taught in the first half of the 2nd/8th century. The great majority of the Ibāḍiyya in the Maghrib at the time looked to him for religious guidance. The teaching activity of ʿAbd Allāh b. Yazīd al-Fazārī among the Maghribīs probably began soon after the death of Abū ʿUbayda sometime between 150/767 and 158/775.

Kitab al-Qadar

Kitāb al-Qadar, a treatise in support of divine determinism against the Muʿtazilī doctrine of human free will.
Al-Fazārī then sets forth the standard arguments of the upholders of divine determinism. Faith inevitably depends on God’s initial favour and His granting of help and success (tawfīq). He does not grant them to those whom He created as unbelievers. The imposition of religious obligations by God on humans and their ability to fulfill them occur at the same moment upon reaching maturity. God does not command anything that is rationally impossible (muḥāl), but He may order something for which He does not grant capability to everybody. Power to act is contemporaneous with the act and does not precede it. God is the provider of all sustenance (rizq) of humans, even if it is stolen, and solely determines their lifespan, even if they are wrongfully killed.
He emphasizes that creation of unbelief and faith are other than unbelief and faith, just as the creation of the heavens and earth are other than the heavens and earth. Then he explains that God’s creation in respect to unbelief and faith, which are human acts, differs from His creation in respect to the heavens, which are made (ṣunʿ ) by God. God’s creation of unbelief and faith merely mean His determination (taqdīr) in the sense of planning them and naming them good or evil. Al-Fazārī then argues vigourously against those who erroneously hold that God created unbelief and faith in the same way as He created the heavens and creates diseases, life and death, quoting numerous relevant statements of the Qurʾān.

Kitāb fi l-radd ʿalā Ibn ʿUmayr

A letter refuting the teaching of Ibn ʿUmayr.
Al-Fazārī accuses him of rather supporting the heretical rationalist views of the Muʿtazila and the radical Shīʿa (Rāfiḍa).
ʿĪsā b. ʿUmayr held that the Muslims had no need for an imam if they faithfully carried out the obligations imposed on them by God. Al-Fazārī insisted that an imam was required to carry out the specific duties reserved for him by the religious law. The theological views upheld by Ibn ʿUmayr reveal a distinct affinity to Donatist Christian theological thought.

Kitāb al-Radd ʿalā al-Mujassima

Al-Fazārī thus refutes here also the Imāmī Shīʿī doctrine of badāʾ.
The second section, however, mostly criticizes traditionalist anthropomorphist doctrine (tashbīh) based on literal acceptance of anthropomorphic description of God in the Qurʾān. At the end of the fragment, al-Fazārī denounces those who hold that God speaks with a tongue and lips. He affirms that God creates His speech without moving. His speech is originated (muḥdath) and created (makhlūq). His criticism here is evidently addressed to the Ḥashwiyya, the Sunnī traditionalists later represented by Aḥmad b. Ḥanbal.

Kitāb al-Futyā

The status of faith of the people in the territory of Islam.
Al-Fazārī then lays down strict rules for recognition of strangers as faithful Muslims which, he notes, go beyond the rules applied generally in the Ibāḍī community. It is evident that he wanted in particular to excommunicate the Qadariyya, upholders of human free will, and radical Khārijīs, who condemned other Muslims as polytheists.

Kitāb al-Tawḥīd fī maʿrifat Allāh

Al-Fazārī affirms that God inspires (alhama) knowledge of Himself in His creatures. Knowledge of God is partly defined (maḥdūd) and partly unlimited. Al-Fazārī then describes the attributes (ṣifāt) and the names (asmāʾ) of God. He distinguishes between divine attributes of essence, attributes of act, and joint (mushtarak) attributes. The sophistication of his discussion suggests that the theory of divine attributes of essence and acts developed in Islamic theology earlier than commonly assumed, well before the time of the famous Muʿtazilī theologians Abu l-Hudhayl and al-Naẓẓām, who flourished in the late 2nd/8th century and are often credited with having initiated such theological thought in their teaching
Al-Fazārī specifically mentions the radical Khārijīs who accuse the rulers and other Muslims of polytheism, the Saba⁠ʾiyya, various Shīʿī groups who consider their imam either as a god, or a prophet, or as an infallible imam whose commands must be obeyed, the Zaydiyya, the Muʿtazila, the Murjiʾa and the Shukkāk. He describes their various heretical views and defines the treatment of heretics in Islam.

Kitāb fī man rajaʿa ʿan ʿilmih wa-fāraqa al-nabī wa-huwa ʿalā dīnih

Al-Fazārī is asked about someone who asserts that the Qurʾān is not created, is he a polytheist? He answers that if he means that the Qurʾān is not originated in time (muḥdath), he has committed an act of polytheism. If he affirms that it is originated in time but will not say that it is created, he is not a polytheist but merely an unbeliever and hypocrite (kāfir munāfiq), like someone who affirms that the acts of humans are not created.