Pronouncing the Intention for Prayer is an Innovation
Question:
What is the ruling on pronouncing the intention to pray aloud?
Answer:
Pronouncing the intention is an innovation, and pronouncing it aloud is an even worse sin. The Sunnah only calls for the intention to be made in the heart, because Allaah, the Most Glorified, Most High knows what is secret and what is hidden, for He says: Say Will you inform Allaah of your religion while Allaah knows all that is in the heavens and all that is in the earth...?
Source:
Shaykh `Abdul-`Azeez Bin Baz
Fatawa Islamiyah Vol. 1 Page 253