Purist Salafism arose in the 20th century, trumpeting a commitment to deriving all law from the Quran and Sunna, and to eliminating polytheism (shirk) and unlawful innovation (bid?a) alike. In its stead, this movement’s scholars articulated an understanding of God’s oneness (Tawhid) that pivoted on the centrality of worship (‘ibada) to daily life. This paper, however, argues that the Salafi conception of worship itself represents a significant rupture with both the early centuries of Islamic jurisprudence and the subsequent Islamic legal tradition because of the way in which it drastically reconfigures the boundaries between this category and that of custom (‘ada/‘urf). This innovation enabled Salafi scholars to offer a comprehensive theological and legal vision that paralleled the claims made by secular-nationalist states and Islamist competitors alike to shape daily practice. To make this argument, the paper draws on an extensive personal archive of Salafi pamphlets, periodicals and legal works produced across the Arab World between 1930 and 1990. It is centered in Egypt, particularly on the country’s leading Salafi organization, Ansar al-Sunna al-Muhammadiyya and its flagship journals, al-Hadi al-Nabawi (1936-69) and al-Tawhid (1973-81, 1983-present). Just as importantly, though, it is engaged with legal and ideological debates by Salafis and non-Salafis across the region, drawing in particular on the Syrian Salafi periodical al-Tamaddun al-Islami (1936-81), the official journal of the Islamic University in Medina (1968-80), and the Kuwaiti Muslim Brotherhood’s al-Mujtama? (1970-92). The argument of this paper is not that Salafis discarded the distinction between devotional acts and custom. Instead, it is that they broadened the domain of worship by redefining the category of custom as a constituent component of worship. It further shows that, while such a conversation had precedence within a longer Islamic legal tradition, the debates of this tradition were not the primary field of contestation. Instead, the move to expand the domain of worship was driven jointly by challenges of colonial rule, post-colonial secular nationalism (and its attendant understanding of customs as a core component of communal identity), and the claim of the Muslim Brotherhood to offer a comprehensive ideological program.