Musnad Ahmad 788:89

And the Messenger of Allah, may Allah bless him and grant him peace, said: "The likeness of that with which Allah, the Mighty and Majestic, has sent me of guidance and knowledge is as the likeness of rain that struck the earth. A portion of it was fertile, so it brought forth fodder and abundant grass. Another portion of it was barren land that held the water, so Allah, the Mighty and Majestic, benefited people through it; they drank, grazed [their livestock], watered [their animals], and cultivated [crops], and they irrigated. And it struck another portion of it. It was merely flat, barren ground that neither held water nor brought forth fodder. That is the likeness of one who gains understanding in the religion of Allah, the Mighty and Majestic, and Allah, the Mighty and Majestic, benefits him with that which He sent me with, so he learns and teaches. And the likeness of one who does not raise his head for that, and does not accept the guidance of Allah, the Mighty and Majestic, with which I was sent."