משנה אויף במדבר 32:16

The legal period of possession [in order to establish ownership] for houses, cisterns, trenches, caves, dovecotes, bath-houses, olive-presses, irrigated fields and slaves and anything which continually produces a yield is three complete years. The legal period of possession [in order to establish ownership] for a field irrigated by rain water is three years and they need not be completed. Rabbi Yishmael says: “Three months during the first year, and three months during the last year and twelve months during the middle year, which makes eighteen months.” Rabbi Akiva says: “One month during the first year and one month during the last year and twelve months during the middle year, which makes fourteen months.” Rabbi Yishmael said: “When does this apply? With regards to a sown field, but with tree plantation, if he brought in his produce (grapes), collected the olives and gathered in his fig harvest, this counts as three years.”