[13].Surah Ar-Ra'd [The Thunder] : Ayat 17

Ayat 17. He sends down water (rain) from the sky, and the valleys flow according to their measure, but the flood bears away the foam that mounts up to the surface, and (also) from that (ore) which they heat in the fire in order to make ornaments or utensils, rises a foam like unto it, thus does Allah (by parables) show forth truth and falsehood . Then, as for the foam it passes away as scum upon the banks, while that which is for the good of mankind remains in the earth. Thus Allah sets forth parables (for the truth and falsehood, i.e. Belief and disbelief). 

Hazrat Abu Musa (May Allah be pleased with him) reported: Messenger of Allah  [SAWW](PBUH) said, "The similitude of guidance and knowledge with which Allah has sent me is like a rain which has fallen on some ground. A fertile part of earth has absorbed water and brought forth much grass and herbs. Another part, which is solid, held the water and Allah benefits men thereby, who drank and gave others to drink, and used it for irrigation. But some of it has fallen on a portion of sandy land which neither retains the water nor produces herbage. Such is the likeness of the man 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 neither raises his head on that account (meaning he does not benefit from what the Prophet  [SAWW](PBUH) was sent with) nor accepts Allah's Guidance with which I am sent".

[Al-Bukhari Book 01, Chapter 03, Hadith # 079]. 

Lesson : as mentioned above in Surah Ar-Ra'd Ayat 17. "He sends down water (rain) from the sky, and the valleys flow according to their measure", In this Hadith the guidance and knowledge which was sent to mankind through the Prophet  [SAWW](PBUH) is compared to the rain which is beneficial, because this knowledge resuscitates the dead hearts as does the rain to a barren land. One who makes use of this knowledge is likened to a good soil, and one who acquires this knowledge and imparts it to others but does not put it into practice is considered akin to a hard land which holds water and other people utilize it. A person who neither learns it nor acts upon it is like a desolate land which neither holds water nor produces anything. Such a person is the worst of all because he neither himself benefits from it nor lets others make use of it. This Hadith induces one to acquire knowledge, impart it to others, put it into actual practice and warns people from neglecting this duty.