Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.018A.

Section : One should seek a soft and low piece of ground to protect himself from impurities.

Abu Musa related that the Messenger of Allah came to a low and soft part of the ground and urinated. He then said, “When one of you urinates, he should choose the proper place to do so.”

This is related by Ahmad and Abu Dawud. One of its narrators is unknown, but its meaning is sound.