Sayyiduna Abu Hurayrah (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) passed by a heap of grain. He (peace and blessings be upon him) put his hand into it, and his (peace and blessings be upon him) fingers felt wetness. He (peace and blessings be upon him) asked: "O owner of the grain! What is this?" He said: O Messenger of Allah! Rain fell on it. He (peace and blessings be upon him) said: "You should have put it on top of the heap so that people could see it. Whoever cheats is not of me."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10733