Maliks Muwatta Book 16, Hadith Number 33.
Section : Stopping for Funerals and Sitting in Graveyards.
Yahya related to me from Malik from Yahya ibn Said from Wafid ibn Amr ibn Said ibn Muadh from Nafi ibn Jubayr ibn Mutim from Masud ibn al-Hakam from Ali ibn Abi Talib that the Messenger of Allah, may Allah bless him and grant him peace, used to stand up when a funeral procession passed by, and then sit down again afterwards.