Abu Dawud
Purification (Kitab Al-Taharah)
Book Number: 001 / Hadith Number: 3
Narated By Abu Musa : 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 Apostle of Allah (pbuh). 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.