Mishkat al-Masabih 345
Abu Musa said:
One day when I was with the Prophet he wanted to pass water, so he went to some soft ground at the foot of a wall and did it, after which he said, "When any of you wishes to pass water, he should look for [a place like this] to do it in.” 

Abu Dawud transmitted it.
وَعَنْ أَبِي مُوسَى قَالَ: كُنْتُ مَعَ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ ذَاتَ يَوْمٍ فَأَرَادَ أَنْ يَبُولَ فَأَتَى دَمِثًا فِي أَصْلِ جِدَارٍ فَبَال ثُمَّ قَالَ: «إِذَا أَرَادَ أَحَدُكُمْ أَنْ يَبُولَ فليرتد لبوله» . رَوَاهُ أَبُو دَاوُد