Sahih Muslim Book 1, Hadith Number 183.

Chapter : The utterance of the Apostle (way peace be upon him). He who acted dishonestly towards us is not of us(i.e. he is not to be counted amongst Muslims).

It is narrated on the authority of Abu Huraira that the Messenger of Allah (may peace be upon him) happened to pass by a heap of eatables (corn). He thrust his hand in that (heap) and his fingers were moistened. He said to the owner of that heap of eatables (corn): What is this? He replied: Messenger of Allah, these have been drenched by rainfall. He (the Holy Prophet) remarked: Why did you not place this (the drenched part of the heap) over other eatables so that the people could see it? He who deceives is not of me (is not my follower).