Musnad Ahmad 1153:298

Ibn Numayr narrated to us, Sharik narrated to us, from al-Miqdam ibn Shurayh al-Harithi, from his father, who said: I said to Aisha: "Did the Prophet, peace and blessings of Allah be upon him, used to go to the desert?" She said: "Yes, he used to go to the desert to these hills. He intended to go to the desert once, so he sent for some camels from the charity camels, and he gave me a she-camel from them that was marked. Then he said: 'O Aisha, you must have fear of Allah, the Almighty and Majestic, and gentleness. For gentleness is never in anything but it adorns it, and it is never removed from anything but it disgraces it.'"