Yahya related to me from Malik that Yahya ibn Said said, "I haveheard that the first of the actions of a slave to be considered on theday of rising is the prayer. If it is accepted from him, the rest ofhis actions will be considered, and if it is not accepted from him,none of his actions will be considered."
USC-MSA web (English) reference: Book 9, Hadith 92
Arabic reference: Book 9, Hadith 424