finding combination of columns which maximizes a quantity

Question

I have a dataframe which looks like this:

  cuid  day  transactions
0    a  mon             1
1    a  tue             2
2    b  tue             2
3    b  wed             1

For each 'cuid' (customer ID) I want to find the day that maximizes the number of transactions. For e.g. for the above df, the output should be a df which looks like

 cuid  day  transactions
1    a  tue             2
2    b  tue             2

I've tried code which looks like:

dd   = {'cuid':['a','a','b','b'],'day':['mon','tue','tue','wed'],'transactions':[1,2,2,1]}
df   = pd.DataFrame(dd)
dfg  = df.groupby(by=['cuid']).agg(transactions=('transactions',max)).reset_index()

But I'm not able to figure out how to join dfg and df.