Musnad Abu Ya'la al-Mawsili 150:67

Muhammad ibn Abdullah ibn Numayr narrated to us, Ibn Abi Fudayk narrated to us, Amr ibn Uthman ibn Hani narrated to me, from al-Qasim ibn Muhammad, from Aisha, she said: I said to her, "O Mother, uncover for me the grave of the Prophet, may Allah bless him and grant him peace, and his two companions." So she uncovered three graves that were level and spread with the red gravel of the courtyard. I saw the Messenger of Allah, may Allah bless him and grant him peace, in front, and Abu Bakr, his head was between the shoulders of the Prophet, may Allah bless him and grant him peace, and Umar, his head was at the feet of the Prophet, may Allah bless him and grant him peace.