'Aayeshah Radiyallahu 'Anha says that:
"After the death of Rasulullah Sallallahu 'Alayhi Wasallam, Abubakr Radiyallahu 'Anhu entered, kissed him on his lower forehead (between the eyes), and put his hands on the shoulders of Rasulullah Sallallahu 'Alayhi Wasallam and said: 'Waa nabiyyaah', He lifted his head bent and kissed the forehead again and said: waa khalilaah.'"
English reference: Book 53, Hadith 374
Arabic reference: Book 54, Hadith 391