Shama-il Tirmidhi Chapter 053, Hadith Number 007 (374).

'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.'"

Commentary
These words were not intended to draw attention, therefore there is no perplexity. A narration in Musnad Ahmad states that Sayyidina Abubakr Radiyallahu 'Anhu came towards the mubaarak head of Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam, bent his head towards the noble face and kissed the forehead, and said: 'Waa nabiyyaah'. He lifted his head bent and kissed the forehead again and said: 'Waa khalilaah".