It is narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: "Whoever comes to relieve himself should conceal himself, and if he does not find anything (to conceal with), he should gather a heap of sand and turn his back towards it, because Satan plays with the backsides of the children of Adam." Whoever does so has done well, and whoever does not, there is no harm upon him.
Hadith Referenceالفتح الربانی / كتاب الطهارة / 493