The Excellence of Intention:
Umar bin al-Khattab Radiallahu anhu said: “The best acts are doing what Allah has commanded, staying for away from what Allah has forbidden, and having sincere intention towards what-ever Allah has required of us.” (Tahdhib ul-Asma)
Some of our predecessors said: “Many small actions are made great by the intentions behind them. Many great actions, on the other hand, are made small because the intentions behind them are lacking.”
Yahya bin Abi Kathir said: “Learn about intentions, for their importance is greater than the importance of actions.”
Ibn ‘Umar once heard a man who was putting on his ihran say: “Allah! I intend to do Hajj and ‘Ummrah.” So he said to him: “is it not in fact the people whom you are informing of your intention? Does not Allah already know what is in your hear?” (Jami’ul-‘Ulum)
It is because good intentions are exclusively the concern of the heart, that they should not be voiced during worship.