It is not prescribed to utter the intention (niyyah) out loud when giving zakaah
Question

                What is the ruling on uttering the intention (niyyah) out loud when giving zakaah? For example, can I say when giving zakaah, “O Allah, this is the zakaah of my wealth”? If that is not permissible, how should one form the intention when giving zakaah?.
Praise be to Allah.
    The place for the intention is the heart, and it is not 
    permissible to utter it out loud for prayer, fasting or zakaah. See the 
    answer to question no. 13337. 

    Shaykh al-Fawzaan (may Allah preserve him) said: … Except in 
    two cases. 

    The first case is when entering ihram for Hajj or ‘Umrah, in 
    which case one says “Labbayka ‘umratan (here I am for ‘Umrah)” or “Labbayka 
    Hajjan (here I am for Hajj)”. 

    The second case is when slaughtering the hadiy or udhiyah or 
    ‘aqeeqah, when one should say Bismillah and state which type it is, whether 
    it is ‘aqeeqah or ‘udhiyah or hadiy, and on whose behalf it is being 
    offered. So he should say: In the name of Allah and on behalf of So and so” 
    or “In the name of Allah on my behalf and on behalf of my household,” then 
    he should slaughter it. 

    In these two cases it is prescribed to utter the intention 
    out loud; in all other cases it is not permissible to utter the intention 
    out loud for any act of worship, prayer or anything else. 

    End quote from al-Muntaqa min Fataawa al-Fawzaan, 
    5/30 

    Based on that, whoever wants to give the zakaah of his wealth 
    should form the intention in his heart that this wealth is the zakaah of his 
    wealth, and it is not prescribed for him to utter his intention verbally. 

    And Allah knows best.