|Know that Tawheed is the first da'wah of the Messengers, the first stopping
place on the path, and the first station in which the pathfinder to ALLAH
(Mighty and Majestic) stands. The Most High said (which means): "Indeed
we sent Noah to his people and he said: 'O my people! Worship ALLAH! You
have no other Ilah (God) but Him." [Al-A'raaf: 59].
And Hud (Peace be upon him) said to his people: "Worship ALLAH! You have no other Ilah (God) but Him." [Al-A'raaf: 65].
And Saalih (Peace be upon him) said to his people: "Worship ALLAH! You have no other Ilah (God) but Him." [Al-A'raaf: 73].
And the Most High said (which means): "And We did not send any Messenger before you (O Muhammad (Peace and Blessings of ALLAH be upon him)) but We inspired him (saying) : 'None has the right to be worshipped but I (ALLAH), so worship Me (Alone and none else)." [Al-Anbiyaa': 25].
And (the Messenger) Peace and Blessings of ALLAH be upon him said: "I have been ordered to fight the people until they testify that there is no ilah but ALLAH, and that Muhammad is the Messenger of ALLAH."(1)
And thus, it is correct that the first waajib (obligation) obligatory on the mukallaf (liable to follow commandments of Shari'ah - every sane, mature, and conscious individual) is the testimony that there is no ilah but ALLAH, so Tawheed is the first obligation and it is the last obligation; as the Prophet (Peace and Blessings of ALLAH be upon him) said: "Whoever's last words were La ilaha ill ALLAH (there is no ilah but ALLAH) will enter Paradise."(2)
(1). Bukhari (25) and Muslim (22) from the hadeeth of Ibn Omar (May ALLAH be pleased with him), and in the chapter from Abu Hurairah, Anas bin Malik, Nu'man bin Basheer, Tariq bin Asheem Al-Ashja'ee, and Mu'adh bin Jabal (May ALLAH be pleased with them all).
(2). Abu Dawud (3116), Ahmad (5/233,247), Al-Haakim (1/351,500) from the hadeeth of Mu'adh bin Jabal, Al-Haakim declared it Saheeh, as did Adh-Dhahabee, and Al-Albanee declared it hasan in Irwaa Al-Ghaleel (3/150)