Sayyiduna Ali bin Abi Talib (may Allah be pleased with him) says: I washed the Prophet (peace and blessings be upon him). I wanted to see the impurity and the like that is usually found with a dead person, but I found nothing. He (peace and blessings be upon him) was pure in life and pure after death. Four men, rather than the people at large, took charge of his (peace and blessings be upon him) burial and of placing him in the grave: Ali, Abbas, Fadl and Salih (may Allah be pleased with them), who was the freed slave of the Messenger of Allah (peace and blessings be upon him). A lahd (niche grave) was dug for the Messenger of Allah (peace and blessings be upon him), and bricks were set up over it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 6626
It is narrated from Sayyiduna Ali (may Allah be pleased with him): I washed the Messenger of Allah (peace and blessings be upon him), and I began looking for what is usually found with a dead person, but I found nothing, for he (peace and blessings be upon him) was pure both in life and after death.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 6627
Ibn Sirin (may Allah have mercy on him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "Whoever washes a dead person should begin by pressing his stomach."
This is mursal, and its narrator is da'if (weak).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 6628