Musnad Ahmad 30:1717

Surayj narrated to us, he said: Isa ibn Yunus narrated to us, from Thawr, from al-Husayn—that is how he said it—from Abu Sa’d al-Khayr—and he was one of the companions of Umar—from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever applies kohl, let him use an odd number. Whoever does so has done well, and whoever does not, there is no blame upon him. Whoever cleans himself with stones (istijmar), let him use an odd number. Whoever does so has done well, and whoever does not, there is no blame upon him. Whoever eats, whatever remains between his teeth, let him cast it out; and whatever he moves with his tongue, let him swallow it. Whoever does so has done well, and whoever does not, there is no blame upon him. Whoever goes to relieve himself, let him conceal himself. If he finds nothing but a mound of sand, let him turn his back to it, for the devil plays with the posteriors of the children of Adam. Whoever does so has done well, and whoever does not, there is no blame upon him."