Mujahid said: My master told me that his family sent with him a bowl of milk and butter for their (pre-Islamic) gods. He took it, and he feared that he might eat some of it, so he set it down. Just then a dog came, drank the milk, ate the butter, and urinated on the idol.
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6587