It was narrated by Abu Musa “The Messenger of ALLAH (PBUH) said: The similitude of guidance and knowledge with which ALLAH has commissioned me is that of abundant rain which has fallen on some ground. A fertile part of the earth, wherefrom has absorbed water and brought forth much grass and herbs. Another part which is hollow, has held the water and ALLAH benefits Men thereby, who have drunk and and gave others to drink, and used it for cultivation . But some of it has not fallen on apportion of barren land which retains the water nor does it produce the herbage. Such is the likeness of he who understands the religion of ALLAH  and who gets benefit of what ALLAH has sent me with; he learns and teaches others. It is also the likeness of the man who neuther raises his head on that account (meaning he does not benefit from what the prophet was sent with) nor accepts ALLAH’s guidance with which I have been sent…. ” [Al-Bukhari and Muslim]
Commentary
In the hadith, the guidance and knowledge  which was sent to Mankind through the prophet (PBUH)is compared to the rain which is beneficial as well as harmfull because this knowledge resuscitates the dead hearts as does the rain to a barren land. One who make use of this knowledge is likened to a good soil, and one who acquired this knowledge and imparts it to others but does not himself put it into practice is considered akin to a hard land which holds water and other people utilize it.