Mukhtasar Bukhari (Abridged) 755 — Ushr is due on crops watered

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."