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.
Sahih Muslim Book 11, Hadith 105
← Previous Hadith Nafi' b. Jubair saw me and we were standing for a bier, while he was sitting and waiting for the bier to be placed on the ground. He said to me: What makes you keep standing? I said: I am waiting that the bier may be placed on the ground (and I am doing that) on the hadith narrated to me by Abu Sa'id al-Khudri. Upon this Nafi' said: Verily, Mas'ud b. Hakam reported to me on the authority of Hadrat 'Ali b. Abu Talib that the Prophet (ﷺ) stood up first (for a bier) and then sat down.

Sahih Muslim Book 11, Hadith 104 Next Hadith → This hadith has been narrated by Yahya b. Sa'id with the same chain of transmitters.

Sahih Muslim Book 11, Hadith 106