From Sahih Muslim (Hadith 214):
It is reported on the authority of Abu Huraira that the Messenger of Allah (may peace and blessings of Allah be upon him) made observations like them embodied in the hadith narrated by Yahya b. Muhammad on the authority of 'Ala', and added to it:even if he observed fast and prayed and asserted that he was a Muslim
Reference: Muslim 214