From Sahih Bukhari (Hadith 1308):
Narrated 'Amir bin Rabi`a:The Prophet (ﷺ) said, "If any one of you sees a funeral procession and he is not going along with it, then he should stand and remain standing till he gets behind it, or it leaves him behind, or the coffin is put down before it goes ahead of him
Reference: Bukhari 1308