Musnad Ahmad 27:624

He said: I found in my father’s book: Ibn Numayr narrated to us, from Hanzalah, from Salim ibn Abdullah, from Ibn Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever acquires a dog, except for a hunting dog or a livestock dog, then two qirats from his reward decrease every day."