Sunan ad-Darimi

It is narrated from Sayyiduna Abu Hurairah, may Allah be pleased with him, that the Messenger of Allah, peace and blessings be upon him, said: "Whoever applies kohl should do so an odd number of times; if he does, it is better, and if he does not, there is no harm. And whoever performs istinja with stones should do so an odd number of times; if he does, it is better, and if he does not, there is no harm. And whoever eats food should use a toothpick, and if something comes out from the toothpick, he should throw it away, and if it sticks to his tongue, he should swallow it; if he does so, it is better, and if he does not, there is no harm. And whoever goes to relieve himself should conceal himself; if there is nothing to conceal him except a heap of sand, then he should sit behind it, because the devils play with a man's private parts. Whoever does so, it is better, and if he does not, there is no harm."

It is narrated from Sayyiduna Abdullah bin Ja'far (may Allah be pleased with him): The Messenger of Allah (peace and blessings be upon him) liked to use a mound or the cover of date-palm trees for concealment at the time of relieving himself.