Find if column contains a certain value in panda

Example I have a dataframe with the following columns:

import pandas as pd
df = pd.DataFrame({
    'A': [1, 4, 7, 1, 4],
    'B': [2, 5, 8, 2, 5],
    'C': [3, 6, 9, 3, 6]
})

Data when represented in Table format is:

   A  B  C
 0
 1
 2
 4
 5

Now I want to find the index of the column that contains the value 5.

How to find the index of column?