Sahih Bukhari Volume 4, Book 55, Hadith 2742

حَدَّثَنَا عَمْرُو بْنُ زُرَارَةَ، أَخْبَرَنَا إِسْمَاعِيلُ، عَنِ ابْنِ عَوْنٍ، عَنْ إِبْرَاهِيمَ، عَنِ الأَسْوَدِ، قَالَ ذَكَرُوا عِنْدَ عَائِشَةَ أَنَّ عَلِيًّا ـ رضى الله عنهما ـ كَانَ وَصِيًّا‏.‏ فَقَالَتْ مَتَى أَوْصَى إِلَيْهِ وَقَدْ كُنْتُ مُسْنِدَتَهُ إِلَى صَدْرِي ـ أَوْ قَالَتْ حَجْرِي ـ فَدَعَا بِالطَّسْتِ، فَلَقَدِ انْخَنَثَ فِي حَجْرِي، فَمَا شَعَرْتُ أَنَّهُ قَدْ مَاتَ، فَمَتَى أَوْصَى إِلَيْهِ

Narrated By Al-Aswad : In the presence of 'Aisha some people mentioned that the Prophet had appointed 'Ali by will as his successor. 'Aisha said, "When did he appoint him by will? Verily when he died he was resting against my chest (or said: in my lap) and he asked for a wash-basin and then collapsed while in that state, and I could not even perceive that he had died, so when did he appoint him by will?"