Hazrat Mutarrif narrates from his father that I saw the Messenger of Allah (peace be upon him) praying, and he spat under his left foot. Hazrat Alaa added that then he rubbed it in.
Hadith Referenceصحيح ابن خزيمه / جماع أبواب الأفعال المباحة فى الصلاة / 879