This article examines how use of the Prophet Muḥammad's Khuṭbat al-ḥāja (Sermon of Necessity) became a distinguishing marker of Salafism. To understand the Sermon's role, the article draws on the notion of "coded language," messages that communities use to communicate with insiders while excluding outsiders. The article analyzes the content of the Sermon and describes its spread among Salafīs. The Sermon was championed by Muḥammad Nāṣir al-Dīn al-Albānī (1914-99), who played a pivotal role in shaping Salafī practice. Relating the Sermon's spread to methodological debates about studying Salafism, the article suggests that the Sermon furnishes one empirical criterion that can be used to date Salafism's crystallization to the mid-twentieth century. The article closes by examining how jihādīs selectively use the Sermon to "Salafize" their speech, and by discussing how instances of opposition to the Sermon's use were connected to debates over the validity of Salafism and the status of al-Albānī. This article examines how use of the Prophet Muḥammad's Khuṭbat al-ḥāja (Sermon of Necessity) became a distinguishing marker of Salafism. To understand the Sermon's role, the article draws on the notion of "coded language," messages that communities use to communicate with insiders while excluding outsiders. The article analyzes the content of the Sermon and describes its spread among Salafīs. The Sermon was championed by Muḥammad Nāṣir al-Dīn al-Albānī (1914-99), who played a pivotal role in shaping Salafī practice. Relating the Sermon's spread to methodological debates about studying Salafism, the article suggests that the Sermon furnishes one empirical criterion that can be used to date Salafism's crystallization to the mid-twentieth century. The article closes by examining how jihādīs selectively use the Sermon to "Salafize" their speech, and by discussing how instances of opposition to the Sermon's use were connected to debates over the validity of Salafism and the status of al-Albānī.