أين ذُكرت كلمة التوحيد (لا إله إلا الله محمد رسول الله) في القرآن الكريم؟

The key to Islam is the two Shahadas, and a person does not enter Islam except through them. It is obligatory to utter them for whoever is able. This specific formulation, "There is no god but Allah, Muhammad is the Messenger of Allah," has been mentioned together in the Sunnah of the Prophet, as in the hadith of Ibn Umar and Mu'adh (may Allah be pleased with them both). As for the Quran, the testimony "There is no god but Allah" and the testimony "Muhammad is the Messenger of Allah" have appeared separately, but they have not appeared together in one place with the well-known formulation.