Sahih Bukhari
Funerals (Al-Janaaiz)
Volume: 002 / Book Number: 023 / Hadith Number: 395
Narrated 'Amir bin Rabi'a:
The Prophet said, "If any one of you see 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 . "