Musnad Abu Ya'la al-Mawsili 209:56

Abu Bakr ibn Abi Shaybah narrated to us, Jarir narrated to us, from al-Mughirah, from Umm Musa, from Umm Salamah, who said: "By the One by whom he swears, Ali was indeed the closest of people in covenant to the Messenger of Allah, may Allah bless him and grant him peace." She said: "The Messenger of Allah, may Allah bless him and grant him peace, was on the day he passed away in the house of Aisha. The Messenger of Allah, may Allah bless him and grant him peace, began saying, morning after morning, 'Has Ali come?' repeatedly." She said: "And I think he had sent him on an errand. Then he came later, and we thought that he had a need for him, so we went out of the house and sat by the door. I was among the closest of them to it. Then Ali leaned over him and began whispering to him and speaking privately with him. Then he passed away on that very day."