Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.119A.

Says Allah, “And We did not command them save to worship Allah, making the religion sincerely for Him” (al-Bayinah 5). The Prophet, upon whom be peace, said, “Every action is based upon intention. For everyone is what he intended. Whoever made the migration to Allah and His Prophet, then his migration is to Allah and His Prophet. Whoever’s migration was for something of this world or for the purpose of marriage, then his migration was to what he migrated to.” (Related by al-Bukhari.)

In Ighatha al-Lufan, Ibn al-Qayyim states, “The intention is the aim and purpose of something. It is a condition of the heart, and it does not come from the tongue. For that reason, the Prophet and his companions never spoke their intentions. What has been introduced into this matter during the actions of purity and the prayer comes from Satan and is a trap for those who are unsure about how to make it. You will find them repeating it over and over, but that is not part of the prayer at all.”