Reference for Zevachim 202:1

וייטב בעיניו לא בוש משה לומר לא שמעתי אלא שמעתי ושכחתי

it was pleasing in his sight'. He admitted his error, and Moses was not ashamed [to excuse himself] by saying, 'I had not heard it', but, 'I heard it and forgot.'

Jastrow