Musnad Abu Ya'la al-Mawsili 150:23

Narrated Ishaq, narrated Sufyan, from Abd Rabbih ibn Sa'id, from Amrah, from Aisha, who said: When a person complained of an ailment, the Prophet (peace and blessings of Allah be upon him) would do this with his saliva, then he would place it upon the earth and say: "The soil of our land, with the saliva of some of us, our sick will be healed, by the permission of our Lord."