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

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.