What does Tawhid mean?
One Creator: “He is Allah; there is no god but He. He is the All-mighty…” (al-Hashr  23).
Only one ruler in Religion: “…He makes none to share in His Decision and His Rule.” (al-Kahf  26).
Only one to pray/call: “And who is further astray than he who calls, apart from God, upon such a one as shall not answer him till the Day of Resurrection? Such as are heedless of their calling?” (al-Ahkaf  5).
Only one source in Religion: “… And when Our signs are recited to them, clear signs, those who look not to encounter Us say, ‘Bring a Quran other than this, or alter it.’ Say: ‘It is not for me to alter it of my own accord. I follow nothing, except what is revealed to me. Truly I fear, if I should rebel against my Lord, the chastisement of a dreadful day.” (Yunus  15).
Only one intercessor: “Say: To Allah belongs all intercession/Shafa’ah…” (al-Zumar  44).
One Religion: “…and I have approved Islam for your religion…” (al-Maida  3).
“…He named you Muslims…” (al-Haj  78).