Musnad Abu Ya'la al-Mawsili 2:86

Abu Kurayb Muhammad ibn al-Ala narrated to us, Waki narrated to us, from Hisham ibn Sa’d, from Zayd ibn Aslam, from his father, from Umar ibn al-Khattab, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The likeness of the one who returns to his charity is like the likeness of the dog that returns to its vomit."