Complex sentence: The "tahlila" (the phrase "lā ilāha illā-llāh") is Quranic, but its combination with the additional "Muhammad is the messenger of God" is of uncertain origin.
Split into simpler sentences: The "tahlila" (the phrase "lā ilāha illā-llāh") is Quranic. But its combination with the additional "Muhammad is the messenger of God" is of uncertain origin.