Ibn Hibban

Sayyiduna Abu Hurairah (may Allah be pleased with him) narrates that the Prophet Muhammad (peace and blessings be upon him) said: “(When performing istinja) whoever uses stones should use them in odd numbers; if he does so, it is good. And whoever goes to relieve himself should cover himself; if he finds nothing but a mound of sand to cover himself, then he should use that, because Satan plays with the private parts of the children of Adam.”