Sahih Muslim , Book 44, Hadith 12 (In-Book) ⓘ

A'isha reported that Allah's Messenger in his (last) illness asked me to call Abu Bakr, her father, and her brother too, so that he might write a document, for he feared that someone else might be desirous (of succeeding him) and that some claimant may say: I have better claim to it, whereas Allah and the Faithful do not substantiate the claim of anyone but that of Abu Bakr.

حَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ سَعِيدٍ، حَدَّثَنَا يَزِيدُ بْنُ هَارُونَ، أَخْبَرَنَا إِبْرَاهِيمُ بْنُ سَعْدٍ، حَدَّثَنَا صَالِحُ بْنُ كَيْسَانَ، عَنِ الزُّهْرِيِّ، عَنْ عُرْوَةَ، عَنْ عَائِشَةَ، قَالَتْ قَالَ لِي رَسُولُ اللَّهِ صلى الله عليه وسلم فِي مَرَضِهِ ‏ "‏ ادْعِي لِي أَبَا بَكْرٍ وَأَخَاكِ حَتَّى أَكْتُبَ كِتَابًا فَإِنِّي أَخَافُ أَنْ يَتَمَنَّى مُتَمَنٍّ وَيَقُولَ قَائِلٌ أَنَا أَوْلَى ‏.‏ وَيَأْبَى اللَّهُ وَالْمُؤْمِنُونَ إِلاَّ أَبَا بَكْرٍ ‏" ‏ ‏.‏