When Adam died, the angels washed his body with water an odd number of times, dug him a grave, and said: This is the tradition for Adam's children
Authentic hadithعن أبي بن كعب، عن النبي صلى الله عليه وسلم قال: «لما تُوُفِّيَ آدمُ غَسَّلَتْه الملائكةُ بالماء وِتْرًا، وأَلْحَدُوا له، وقالوا: هذه سُنَّةُ آدَمَ في وَلَدِه».
Ubayy ibn Ka‘b (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "When Adam died, the angels washed his body with water an odd number of times, dug him a grave, and said: 'This is the tradition for Adam's children.'"
Explanation
When Adam (peace be upon him) died, the angels washed his body with water an odd number of times, which could be one, three, or five times, and dug a hole in the side of the grave and buried him therein. Then they said: "This is the tradition for Adam's children." In other words, the children of Adam should go through the same procedure of washing and burial that their father went through. Indeed, only the guided among Adam's children will abide by this tradition.
Source · Al-Haakim - At-Tabaraani
From the Hadith Encyclopedia (hadeethenc.com).