Sunan Abi Dawud 3
Abu al-Tayyah reported on the authority of a shaykh (an old man):
When Abdullah ibn Abbas came to Basrah, people narrated to him traditions from AbuMusa.  Therefore Ibn Abbas wrote to him asking him about certain things.  In reply AbuMusa wrote to him saying: One day I was in the company of the Messenger of Allah (ﷺ).  He wanted to urinate.  Then he came to a soft ground at the foot of a wall and urinated.  He (the Prophet) then said: If any of you wants to urinate, he should look for a place (like this) for his urination.
حَدَّثَنَا مُوسَى بْنُ إِسْمَاعِيلَ، حَدَّثَنَا حَمَّادٌ، أَخْبَرَنَا أَبُو التَّيَّاحِ، حَدَّثَنِي شَيْخٌ، قَالَ لَمَّا قَدِمَ عَبْدُ اللَّهِ بْنُ عَبَّاسٍ الْبَصْرَةَ فَكَانَ يُحَدَّثُ عَنْ أَبِي مُوسَى، فَكَتَبَ عَبْدُ اللَّهِ إِلَى أَبِي مُوسَى يَسْأَلُهُ عَنْ أَشْيَاءَ، فَكَتَبَ إِلَيْهِ أَبُو مُوسَى إِنِّي كُنْتُ مَعَ رَسُولِ اللَّهِ صلى الله عليه وسلم ذَاتَ يَوْمٍ فَأَرَادَ أَنْ يَبُولَ فَأَتَى دَمِثًا فِي أَصْلِ جِدَارٍ فَبَالَ ثُمَّ قَالَ صلى الله عليه وسلم ‏
"‏ إِذَا أَرَادَ أَحَدُكُمْ أَنْ يَبُولَ فَلْيَرْتَدْ لِبَوْلِهِ مَوْضِعًا ‏"‏ ‏.‏