Ibn 'Abbaas Radiyallahu 'Anhu and 'Aayeshah Radiyallahu 'Anha report:
"After the death of Rasulullah Sallallahu 'Alayhi Wasallam, Abubakr Siddiqe Radiyallahu 'Anhu came and kissed the forehead of Rasuluilah Sallallahu 'Alayhi Wasallam". (PT)
English reference (PT): Book 53, Hadith 373 (PT)
Arabic reference (PT): Book 54, Hadith 390 (PT)