||'That would hardly be suitable,' replied Moses. 'What we will sacrifice to God our Lord is sacred to the Egyptians. Could we sacrifice the sacred animal of the Egyptians before their very eyes and not have them stone us?
Vayomer Moshe lo nachon la'asot ken ki to'avat Mitsrayim nizbach l'Adonay Eloheynu hen nizbach et-to'avat Mitsrayim le'eyneyhem velo yiskelunu.
(Targum Yonathan; Rashi). See notes of Genesis 39:6, 43:32. Others say that the sheep was disgusting to the Egyptians (Rashbam; cf. Ibn Ezra).