Question 15: Is it permissible to utter the intention verbally? For example, on slaughtering an Ud-hiyah (sacrificial animal offered by non-pilgrims) on behalf of my dead father, may I say: ‘O Allaah, this is the Ud-hiyah of my father so-and-so’? Or, does it suffice to do any act without uttering the intention?
Answer: The place of the intention is in the heart. Thus, what is intended in the heart is sufficient and there is no need to utter the intention verbally. Rather, one has to say (“Bismillaah [In the Name of Allaah]”) and Takbeer (“Allaahu Akbar [Allaah is the Greatest]”) when slaughtering… read more here.