Praise be to Allah.
The first duty enjoined upon mankind is the first thing to which people should be called. The Prophet (peace and blessings of Allaah be upon him) explained that to Mu’aadh ibn Jabal (may Allaah be pleased with him) when he sent him to Yemen. He said to him: “You will come to some of the People of the Book, so let the first thing to which you call them be the testimony that there is no god but Allaah and that Muhammad is His Messenger.” This is the first duty enjoined upon people, to believe in Allaah alone and to bear witness that Muhammad (peace and blessings of Allaah be upon him) is his Messenger. By affirming the Unity of Allaah (Tawheed) and bearing witness to His Messenger (peace and blessings of Allaah be upon him), one attains sincerity (ikhlaas) and obedience (in accordance with what is prescribed in sharee’ah), and these are the conditions of all acts of worship being accepted.