4

On a ('alā is semantically connected to the preceding [statement]) straight path, that is, [you follow] the way of the prophets before you, [enjoining] the affirmation of Allah’s Oneness and guidance (the emphasis expressed by the oath [in 'by the definitive Qur'ān'] and the remainder [of the statement] is a response to the disbelievers' saying to him, 'You have not been sent [by Allahu ta’ālā]!' [Q. 13:43].