Abu Dawud Book 1, Hadith Number 25.
Chapter : Places where urinating is prohibited.
Narated By Abu Hurairah : The Prophet (PBUH) said: Be on your guard against two things which provoke cursing. They (the hearers) said: Prophet of Allah, what are these things which provoke cursing? He said: Easing in thoroughfares (where people walk) or under the shade (of the trees) (where they take shelter and rest).