Hadith #2114 (Sahih Muslim)
Book: The Book of Prayer - Funerals
Narrator(s): Mas'ud bin Jwyryh bin Da'ud مسعود بن جويرية بن داود
Grade: Sahih

Verily, the Prophet (ﷺ) used to stand first but later on kept sitting; but it is also narrated that Nafi’ ibn Jubair saw Waqid b. ‘Amr standing for a bier till it was placed down.

Source: https://parallelquran.com/hadith/sahih-muslim-2114/