Abdullah bin Abbas reported,
When the people resolved to wash the body of the Messenger of Allah ﷺ, no one was in the house except his family, his uncle Abbas bin Abdul-Muttalib, Ali bin Abu Talib, Fadl bin Abbas, Qutham bin Abbas, Usama bin Zayd bin Haritha, and his freedman Salih.When they had resolved to wash him, Aws bin Khawli al-Ansari, one of the Banu Awf bin Khazraj and a veteran of Badr, called out from behind the door to Ali bin Abu Talib, saying, "Ali, I implore you by Allah, give us our share of the Messenger of Allah ﷺ."Ali said to him, "Come in."So he entered and was present at the washing of the Messenger of Allah ﷺ, though he took no part in the actual washing.Ali leaned the body against his chest, with the shirt still upon him. Abbas, Fadl, and Qutham turned the body with Ali bin Abu Talib, while Usama bin Zayd and their freedman Salih poured the water. Ali washed him, and nothing was seen from the Messenger of Allah ﷺ of what is usually seen from a deceased person. Ali kept saying, "May my father and mother be sacrificed for you. How fragrant you are, in life and in death."
Azami 9370
Hasanعن ابن عباس قال
لما أجمع القوم لغسل رسول الله ﷺ وليس في البيت إلا أهله: عمه العباس بن عبد المطلب، وعلي بن أبي طالب، والفضل بن العباس، وقثم بن العباس، وأسامة بن زيد بن حارثة، وصالح مولاه، فلما أجمعوا الغسل نادى من وراء الباب أوس بن خولي الأنصاري- ثم أحد بني عوف بن الخزرج، وكان بدريا- علي بن أبي طالب فقال له: يا علي نشدتك الله وحظنا من رسول الله ﷺ. قال: فقال له علي: ادخل. فدخل فحضر غسل رسول الله ﷺ ولم يل من غسله شيئا. قال: فأسنده إلى صدره، وعليه قميصه، وكان العباس والفضل وقثم يقلبونه مع علي بن أبي طالب، وكان أسامة بن زيد وصالح مولاهما يصبان الماء، وجعل علي يغسله، ولم ير من رسول الله ﷺ شيء مما يرى من الميت، وهو يقول: بأبي وأمي ما أطيبك حيا وميتا!
Ahmad narrated it (2357) from Yaqub, who said, my father narrated to us, from Ibn Ishaq, who said, Husayn bin Abdullah narrated to me, from Ikrima, from Ibn Abbas, and he mentioned it. Husayn bin Abdullah is daif. But he was corroborated, Abdullah bin Abu Bakr corroborated him, as Ibn Ishaq narrated it, saying,When Abu Bakr 
was given the pledge of allegiance, the people turned to preparing the body of the Messenger of Allah ﷺ on Tuesday. Abdullah bin Abu Bakr, Husayn bin Abdullah, and others among our companions narrated to me that Ali bin Abu Talib and Abbas bin Abdul-Muttalib, and he mentioned the number of those who undertook the washing of the Messenger of Allah's ﷺ body.This is how Ibn Hisham mentioned it in his Sirah (2/662) as a mursal narration, and Ibn Jareer Tabari narrated it in his Tarikh (3/211) as a musnad narration, from someone who reported to him from Abdullah bin Abbas. The unnamed person is Ikrima, as appears in Ahmad's isnad.With these two chains, the hadith rises to the level of hasan.