Musannaf Ibn Abi Shayba 36:501

Yahya ibn Sa'id narrated to us, from Sufyan, from Musa ibn Abi 'Aisha, from 'Ubayd Allah ibn 'Abd Allah ibn 'Utbah, from 'Aishah and Ibn 'Abbas, that Abu Bakr kissed the Prophet, may Allah bless him and grant him peace, after he had died.