Ibn Abi Mulaykah (may Allah have mercy on him) says that once Sayyiduna Abu Bakr al-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 / 64
Hadith Gradingحکم دارالسلام:إسناده ضعيف لانقطاعه ، فإن ابن أبى مليكة لم يدرك أبا بكر