R assign order references by other column value

Question

I have this df:

 BU  Others
 IT   10
 IT   11
 IT   10
 ING  11
 ING  10
 ING  11

The df have been ordered by "BU".

I would like to assign another column with a reference number depends on the number of row for each value of "BU".

Like that:

 BU  Others  Reference
 IT   10      1
 IT   11      2
 IT   10      3
 ING  11      1
 ING  10      2
 ING  11      3

I´ve tried something like that (with mutate, transform, creating the column previously...), but not working:

df<-df %>% 
 mutate(Reference=ifelse(BU== shift(BU),shift(Reference)+1,1))