Reference for Megillah 12:3

ואמר ר' יצחק אם יאמר לך אדם יגעתי ולא מצאתי אל תאמן לא יגעתי ומצאתי אל תאמן יגעתי ומצאתי תאמן

Rabbi Yitzhak said: If a person says to you, 'I have labored but I did not find', do not believe [him]. [If he says,] 'I did not labor, but I have found', do not believe [him]. [If he says,] 'I have labored and I have found', believe [him].

Jastrow