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