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". (ES)
English reference (ES): Book 53, Hadith 373 (ES)
Arabic reference (ES): Book 54, Hadith 390 (ES)