Is the Prophet's — peace and blessings be upon him — request to his uncle Abu Talib to say "Lā ilāha illā Allāh" (There is no god but Allah) a justification for saying that this testimony alone is sufficient without "Muhammadun Rasūl Allāh" (Muhammad is the Messenger of Allah), and how do we reconcile this with the verses indicating the obligation of believing in all messengers?

Belief in the prophethood of Muhammad (peace and blessings be upon him) and following him is a fundamental pillar for the validity of Islam. To claim otherwise is an act of disbelief that expels its speaker from Islam.

If belief in Allah alone, or the testimony "there is no god but Allah," is mentioned singularly, it implicitly includes belief in the prophethood of Muhammad (peace and blessings be upon him), and faith is not valid without it.

Citing verses and hadiths that mention belief in Allah or the testimony of monotheism without explicitly mentioning the prophethood of Muhammad (peace and blessings be upon him) does not mean that belief in him is not obligatory. Rather, it is encompassed within that, just like all the pillars of faith which may not all be mentioned in a single text.

Merely citing evidence for a corrupt statement does not give it strength, for many proponents of falsehood cite evidence for their falsehoods.