The Shahadah (Arabic: ٱلشَّهَادَة, audio (help·info)) is the first pillar of Islam. The saying is "Lā ʾIlāha ʾIllā Allah, Muḥammadun Rasūl Allah", meaning "There is no deity but God, Muhammad is the messenger of God".
In one of the many ayahs (proofs, evidence, verses, signs, revelations, etc.) within the the Quran. The ayah says 3:18
شَهِدَ ٱللَّهُ أَنَّهُۥ لَآ إِلَـٰهَ إِلَّا هُوَ وَٱلْمَلَـٰٓئِكَةُ وَأُو۟لُوا۟ ٱلْعِلْمِ قَآئِمًۢا بِٱلْقِسْطِ ۚ لَآ إِلَـٰهَ إِلَّا هُوَ ٱلْعَزِيزُ ٱلْحَكِيمُ ١٨
Allâh bears witness that Lâ ilâha illa Huwa (none has the right to be worshipped but He), and the angels, and those having knowledge (also give this witness); (He always) maintains His creation in Justice. Lâ ilâha illa Huwa (none has the right to be worshipped but He), the All-Mighty, the All-Wise.