Reference for Yevamot 156:19
ויאמר ה' אל שאול ואל בית הדמים על אשר המית הגבעונים אל שאול שלא נספד כהלכה ואל בית הדמים על אשר המית הגבעונים וכי היכן מצינו בשאול שהמית הגבעונים אלא מתוך שהרג נוב עיר הכהנים שהיו מספיקין להם מים ומזון מעלה עליו הכתוב כאילו הרגן
In the first year he said to them, 'It is possible that there are idolaters among you, for it is written, And serve other gods, and worship them … and he will shut up the heaven, so that there shall be no rain etc.'<span class="x" onmousemove="('comment',' Deut. XI. 16f. ');"><sup>47</sup></span> They instituted enquiries but could not discover any idolaters. In the second year he said to them, 'There may be transgressors among you, for it is written, Therefore the showers have been withheld and there hath been no latter rain; yet thou hadst a harlot's forehead etc.'<span class="x" onmousemove="('comment',' Jer. III, 3. ');"><sup>48</sup></span> Enquiries were made but none was found. In the third year he said to them, 'There might be among you men who announce specified sums for charity in public but do not give them, as it is written, As vapours and wind without rain, so is he that boasteth himself of a false gift'.<span class="x" onmousemove="('comment',' Prov. XXV, 14. ');"><sup>49</sup></span>