Lucius is saying his goodbyes at Platform 9 ¾ before Narcissa returns to school.
Lucius: Now you must promise me that you won’t fall in love with anyone else while you’re back at school.
Narcissa: Are you assuming that I’m in love with you?
Narcissa: Because I am. I just don’t appreciate the assumption.
Narcissa: *kisses his cheek before turning to board the train*
Lucius: You must promise to write to me every day!
Narcissa: Not on your life, Malfoy.