In The Name of Allaah, The Most Merciful, The Bestower of Mercy.

 Uttering one’s intention at the time of performing ablution

It is not allowed. The place of intention is the heart. Uttering the intention loud is something that was not done by our Prophet, and he is the example to be followed.  The Niyyah [legislated intention] is that the performer of ablution establishes in his heart that such ablution is for the prayer, or (for) touching a copy of the Qur’aan, or for [purification after one’s ablution has been nullified by defecation, urination or passing wind etc.] and what is similar to it. This is intention, because intention is the conviction of the heart to carry out an act of worship. The Prophet [sallal-laahu-alayhi-wasallam] ordered us to begin the ablution with the Basmalah [i.e. to say بِالسْمِ اللّه] and nothing other than it. So to begin it [i.e. the ablution] by pronouncing one’s intention loud is in opposition to what he [sallal-laahu-alayhi-wsallam] has indicated to and commanded. [Source: Al-Min’dhaar Fee Bayaan Katheer Minal Akhtaa Ash-Shaa’i’ah. page 20]