Musnad Abu Ya'la al-Mawsili 160:125

And with his chain of narration: That the Messenger of Allah (may Allah bless him and grant him peace) passed by a heap of food. He inserted his hand into it, and his fingers touched wetness. He said: "What is this, O owner of the food?" He said: "The rain (sky) afflicted it, O Messenger of Allah." He said: "Why did you not place it on top of the food so that the people could see it? Whoever cheats me is not of me."