Midrash sur Ta'anit 3:10

Rabbi Phineas said: Thirty years after Saul and his sons had been killed, a famine lasting three years arose in the days of David, year after year, as it is said, "And there was a famine in the days of David three years, year after year" (2 Sam. 21:1). Why was it year after year? In the first year all Israel went up to (celebrate the great) festivals. David said to them: Go and look if perchance there be among you some who worship idols, for because of the sin of idolatry rain is withheld, as it is said, "Take heed to yourselves, lest your heart be deceived, and ye turn aside, and serve other gods, and worship them" (Deut. 11:16). What is written after this? "And the anger of the Lord will be kindled against you, and he will shut up the heaven, that there be no rain" (Deut. 11:17). They went forth and investigated, but did not find (any idolatry).