اللّهُـمَّ بارِكْ لَنا فيهِ وَأَطْـعِمْنا خَـيْراً مِنْـه/ اللّهُـمَّ بارِكْ لَنا فيهِ وَزِدْنا مِنْه
Allaahumma baarik lanaa feehi wa zidnaa minhu. / Allaahumma baarik lanaafeehi wa at imnaa khayranminhu.
O Allah, bless us in it and provide us with better than it.
Whomever Allah has given milk to drink, should say :
O Allah, bless us in it and give us more of it.
Reference: At-Tirmithi 5/506. See also Al-Albani, Sahih At-Tirmithi 3/158.