Sahih Muslim 1823 a
It has been narrated on the authority of 'Abdullah b. 'Umar who said:
I was present with my father when he was wounded. People praised him and said: May God give you a noble recompense! He said: I am hopeful (of God's mercy) as well as afraid (of His wrath) People said: Appoint anyone as your successor. He said: Should I carry the burden of conducting your affairs in my life as well as in my death? (So far as Caliphate is concerned) I wish I could acquit myself (before the Almighty) in a way that there is neither anything to my credit nor anything to my discredit. If I would appoint my successor, (I would because) one better than me did so. (He meant Abu Bakr.) If I would leave You alone, (I would do so because) one better than me, i. e. the Messenger of Allah (ﷺ), did so. 'Abdullah says: When he mentioned the Messenger of Allah (ﷺ) I understood that he would not appoint anyone as Caliph.
حَدَّثَنَا أَبُو كُرَيْبٍ، مُحَمَّدُ بْنُ الْعَلاَءِ حَدَّثَنَا أَبُو أُسَامَةَ، عَنْ هِشَامِ بْنِ عُرْوَةَ، عَنْ أَبِيهِ، عَنِ ابْنِ عُمَرَ، قَالَ حَضَرْتُ أَبِي حِينَ أُصِيبَ فَأَثْنَوْا عَلَيْهِ وَقَالُوا جَزَاكَ اللَّهُ خَيْرًا ‏.‏ فَقَالَ رَاغِبٌ وَرَاهِبٌ قَالُوا اسْتَخْلِفْ فَقَالَ أَتَحَمَّلُ أَمْرَكُمْ حَيًّا وَمَيِّتًا لَوَدِدْتُ أَنَّ حَظِّي مِنْهَا الْكَفَافُ لاَ عَلَىَّ وَلاَ لِي فَإِنْ أَسْتَخْلِفْ فَقَدِ اسْتَخْلَفَ مَنْ هُوَ خَيْرٌ مِنِّي - يَعْنِي أَبَا بَكْرٍ - وَإِنْ أَتْرُكْكُمْ فَقَدْ تَرَكَكُمْ مَنْ هُوَ خَيْرٌ مِنِّي رَسُولُ اللَّهِ صلى الله عليه وسلم ‏.‏ قَالَ عَبْدُ اللَّهِ فَعَرَفْتُ أَنَّهُ حِينَ ذَكَرَ رَسُولَ اللَّهِ صلى الله عليه وسلم غَيْرُ مُسْتَخْلِفٍ ‏.‏