Musnad Ahmad 1153:2119

'Abd al-Samad and Mu'awiyah ibn 'Amr narrated to us, they said: Za'idah narrated to us, Musa ibn Abi 'A'ishah narrated to us, from 'Ubayd Allah ibn 'Abd Allah, who said: I entered upon 'A'ishah and said to her: "Will you not tell me about the illness of the Messenger of Allah, may Allah bless him and grant him peace?" She said: "Yes. The Messenger of Allah, may Allah bless him and grant him peace, became heavy." Then she mentioned the hadith, and he said: "The Messenger of Allah, may Allah bless him and grant him peace, gestured to him not to delay." Mu'awiyah said: "To delay." And he said to them: "Seat me beside him." So they seated him beside him. She said: "Abu Bakr began to pray while standing, following the prayer of the Messenger of Allah, may Allah bless him and grant him peace, and the people were praying following the prayer of Abu Bakr, while the Prophet, may Allah bless him and grant him peace, was sitting."