Ibn Abi Mulaykah (may Allah have mercy on him) says that once Sayyiduna Abu Bakr Siddiq (may Allah be pleased with him) was addressed as "O Caliph of Allah," so he said: I am not the Caliph of Allah, rather I am the Caliph of the Messenger of Allah, and I am content with that rank.
Hadith Referenceمسند احمد / مسند الخلفاء الراشدين / 59
Hadith Gradingحکم دارالسلام:إسناده ضعيف لانقطاعه، فإن ابن أبى مليكة لم يدرك أبا بكر