Saying, 'There is no God but Allah' in a funeral procession
The basic ruling of acts of worship is that they are Tawqifiy (bound by a religious text and not amenable to personal opinion) because of the saying of the Prophet (peace be upon him):
Anyone who introduces anything into this matter of ours (Islam) that is not part of it will have it rejected.
Agreed upon Hadith.
According to the narration of
Muslim, the Hadith states:
Anyone who does an action which is not stated in this matter of ours (Islam) will have it rejected.
The Sunnah of the Prophet (peace be upon him) regarding offering Funeral Prayer, processing the dead and burying it is well-known to Muslims. However, the Prophet (peace be upon him) did not say: "La ilaha illa Allah" in the funeral procession and all goodness is in following the Prophet (peace be upon him). (Part No. 9; Page No. 21) May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.