Interesting question about where it says God hears our thoughts. So I did a search and found a surprising number of verses where this is stated:
1 Chron 28:9- And thou, Solomon my son, know thou the God of thy father, and serve him with a perfect heart and with a willing mind: for the LORD searcheth all hearts, and understandeth all the imaginations of the thoughts : if thou seek him, he will be found of thee; but if thou forsake him, he will cast thee off for ever. KJV
Ps 94:11- The LORD knoweth the thoughts of man, that they are vanity. KJV
Ps 139:23- Search me, O God, and know my heart: try me, and know my thoughts : KJV
Isa 66:18- 8 For I know their works and their thoughts... KJV
Prov 15:26- 6 The thoughts of the wicked are an abomination to the LORD... KJV
Matt 9:4- And Jesus knowing their thoughts said, Wherefore think ye evil in your hearts? KJV
Luke 5:22- But when Jesus perceived their thoughts , he answering said unto them, What reason ye in your hearts? KJV
Luke 6:8- But he knew their thoughts, and said to the man which had the withered hand, Rise up, and stand forth in the midst. And he arose and stood forth. KJV
Luke 11:17- But he, knowing their thoughts, said unto them, Every kingdom divided against itself is brought to desolation; and a house divided against a house falleth. KJV
Heb 4:12- 2 For the word of God is quick, and powerful, and sharper than any twoedged sword, piercing even to the dividing asunder of soul and spirit, and of the joints and marrow, and is a discerner of the thoughts and intents of the heart. KJV
Yes, God knows the thoughts and intents of your heart, but spoken prayer has a different kind of power behind it simply because there is power in words. Words are how God created the heavens and the earth (God said... and it was); He creates the fruit of our lips (Is. 57:19); and He specifically said to Put Me in remembrance... declare thou that thou mayest be justified. (Is. 43:26). Take with you words and return to the Lord (Hos. 14:2). You shall decree a thing and it shall be established for you and the light of God’s favor shall shine on your ways. (Job 22:28). Praying the Word out loud has much more power than thought prayer. We are to return the Word of God back to Him in prayer for our benefit, not his. It actually puts us in remembrance of His promises to us, and builds our faith. And faith is what brings prayer results. Spoken prayer that speaks the Word has power.