Matthew 18:12. If a man have an hundred sheep, and one of them be gone astray, doth he not leave the ninety and nine, and goeth into the mountains, and seeketh that which is gone astray?
See also Luke 15:4,7.
This mode of expression was very common with the Jews. Lightfoot cites from the Talmud a passage illustrative of it: “When a man is dividing nuts among the poor, though ninety-nine call upon him to divide them, and one call on him to scatter them, to him they must hearken. With grapes and dates it is not so. Though ninety-nine call on him to scatter them, and one to divide them, to him they must hearken.”