Musnad Ahmad 1215:1

Husayn ibn Muhammad narrated to us, he said: Yazid—meaning Ibn ‘Ata’—narrated to us, from Yazid—meaning Ibn Abi Ziyad—from Sulayman ibn ‘Amr ibn al-Ahwas al-Azdi, who said: My mother narrated to me that she saw the Messenger of Allah (peace and blessings of Allah be upon him) stoning Jamrat al-‘Aqabah from the bottom of the valley, and behind him was a person shielding him from the people so that they would not hit him with stones, while he was saying: "O people, do not kill one another, and when you throw, throw with pebbles the size of a sling-stone." Then he turned, and a woman came to him with a son of hers and said: "O Messenger of Allah, this son of mine is mentally deficient, so supplicate to Allah for him." He said to her: "Bring me some water." She brought him water in a stone vessel. He spat into it, washed his face, and then supplicated over it. Then he said: "Go, wash him with it, and seek healing from Allah, the Mighty and Majestic." I said to her: "Give me a little of it for this son of mine." So I took a little of it with my fingers and wiped the side of my son with it, and he became one of the most righteous of people. I asked the woman afterward: "What happened to her son?" She said: "He recovered in the best of recoveries."