From Sahih Muslim (Hadith 2230):
It is narrated on the authority of Muhammad b. Munkadir that he said:I heard from Mas'ud b. al-Hakam who narrated it on the authority of Hadrat 'Ali that he said: We saw the Prophet (ﷺ) stood up for a (bier) and we also stood up; he sat down and we too sat down
Reference: Muslim 2230