Al-Hasan (may Allah have mercy on him) said: A man says, "By Allah! I did not do it," when he had in fact done it forgetfully; that is nothing. It is a lie; let him seek Allah's forgiveness, and there is no expiation on him.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 19945