Imaam al-Kaasaani
رحمة الله عليه, one of the giants of the Hanafi Madh-hab, mentions in
Badaa'i`-us-Sanaa'i` fee Tarteeb-ish-Sharaa'i` that the place of the intention (
Niyyah) is the heart; hence, even if no verbal intention is made, your
Niyyah is valid. However, it is
Mustahabb (recommended) to make the intention verbally along with making it in the heart.
والله تعالى أعلم