In The Name of Allaah, The Most Merciful, The Bestower of Mercy
Shaikh Saaleh Al-Fawzaan [may Allaah preserve him]
And due to the precedence given to this obligation [i.e. to worship Allaah alone and not to associate anyone or anything with Him in worship] over all other obligations, and since it is the basis of all the rulings of the religion, we find that the Prophet [sallal laahu alayhi wasallam] stayed in Makkah for thirteen years -calling the people to worship Allaah alone and negated all partners associated with Allaah in worship. The majority of the Aayaat of the Noble Qur’aan established this affair of Tawheed and negated all the doubts held by the people regarding it. Every performer of the prayer, whether whilst performing an obligatory or optional prayer, has made a covenant with Allaah that he [or she] will establish this affair when he [or she] says: [إِيَّاكَ نَعۡبُدُ وَإِيَّاكَ نَسۡتَعِينُ – You (Alone) we worship, and You (Alone) we ask for help (for each and everything).
This great obligation is called Tawheed Al-Ibaadah [i.e. to single out Allaah in worship]
Source: Muhaadaraat Fil Aqeedah Wad-Da’wah’ vol 1′ pg 8. Abridged & slightly paraphrased