Combining multiple columns in one R

How can I combine multiple all dataframe's columns in just 1 column? , in an efficient way... I mean not using the column names to do it, using dplyr or tidyr on R, cause I have too much columns (10.000+)

For example, converting this data frame

  > Multiple_dataframe
      a    b    c

      1    4    7
      2    5    8
      3    6    9

to

  > Uni_dataframe
     d

     1
     2
     3
     4
     5
     6
     7
     8
     9

I looked around Stack Overflow but without success.