Mukhtasar Bukhari

Narrated by Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) from the Noble Prophet (peace and blessings be upon him) that he said: "On that which is irrigated by the water of the sky, i.e., rain, or by springs, or grows by itself, one-tenth (ushr) is obligatory, and on that which is irrigated by well water, half of one-tenth (i.e., one-twentieth) is obligatory."