In his lessons on Nawāqiḍ al-Islām, the Shaykh was asked:

Are the Khārijites considered from the people of the qiblah (i.e. Muslims) and are they to be prayed behind? What is the principle for praying behind a person from the people of the qiblah?

Shaykh Ṣāliḥ al-Fawzān answered:

The scholars have differed about the Khārijites, are they disbelievers or are they misguided and sinful – upon two sayings. The saying that they are disbelievers [outside the fold of Islām] is closer to [the truth] because the evidences indicate their disbelief. As for prayer behind them, it is not permissible built upon them being disbelievers except when they have taken over a land as has been mentioned by the jurists. In that case, the Muslim prays behind them and does not abandon the congregational prayer.

Reference: Durūs Fī Sharḥ Nawāqiḍ al-Islām (1425H) p. 170.

Note that takfīr of the Khārijites has also been indicated by Imām al-Bukhārī, al-Qāḍī Abu Bakr, al-Subkī, al-Qurṭubī, al-Qāḍī ʿIyāḍ, al-Nawawī and likewise Imām Ibn Bāz. Shaykh al-Fawzān has mentioned above that scholars have differed in this regard and in the answer above, the Shaykh states that the view closer to the truth is that the Khārijtes are disbelievers. The Shaykh also has other verdicts in which he tends to the view that they are misguided sinners and not disbelievers.