四分比丘戒本疏 2.126

2.126

三十八、含飯者，《僧祇》云「師長喚時，咽未盡，能使聲不異者得應；若不得應，咽已應。若前人嫌者，應答口中有食。」

Rule thirty-eight, with a mouthful of rice: the Mahāsāṃghika-vinaya says: "When teachers or elders call, if swallowing is not yet finished, if one can make one's voice sound normal, one may reply; if one cannot, reply after swallowing. If the other person is displeased, one should answer: 'There was food in my mouth.'"