Musnad Abu Ya'la al-Mawsili 42:125

Al-Abbas ibn al-Walid narrated to us, Abd al-Wahid ibn Ziyad narrated to us, from al-A'mash, from Abu Salih, from Abu Sa'id, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Allah’s mercy consists of one hundred parts. He distributed one part of it among the creation; through it, people, wild animals, and birds show mercy to one another."