The intention is that the Prophet (Sallallahu Alaihi Wa Sallam) also used to purify himself with water after using the mud etc. after that he would first rub his hands on the ground and wash them and then perform WUDU (ablution). As is clear from this tradition that it was usual with the Prophet (Sallallahu Alaihi Wa Sallam) that he would perform WUDU (ablution) after answering a call of nature and doing ISTINJA. But in order to indicate that performing WUDU (ablution) is only a commendable and excellent practice he has occasionally omitted it, as is related in Sunan Abu Dawud and Sunan Ibn MAJAH on the basis of the following traditions from Hadrat Aishah.
Once after the Prophet (Sallallahu Alaihi Wa Sallam) had passed urine Hadrat Umer brought a jug of water. The Prophet (Sallallahu AIaihi Wa Sallam) thereupon asked: O’mar! What is this water for? Omar replied, Water for your Wudu. The Prophet (Sallallahu Alaihi Wa Sallam) remarked, I have not been required to perform Wudu each time after I have passed water, If I were do so, it will become a sunnah and an established practice. [Ma’ruf -ul- hadis]