Musnad Abu Ya'la al-Mawsili 1:27

Al-Qawariri narrated to us, Yahya bin Sa'id narrated to us, from Sufyan, from Musa bin Abi 'Aishah, from 'Ubaydullah bin 'Abdullah, from Ibn 'Abbas and 'Aishah: That Abu Bakr kissed the Prophet, may Allah bless him and grant him peace, while he was dead.