From Longman Dictionary of Contemporary Englishin writingin writingWRITEif you get something in writing, it is official proof of an agreement, promise etc Could you put that in writing, please? → writing
Examples from the Corpus
in writing• You should have asked them to put what they agreed in writing.• I don't have anything in writing, but they said they expected me to start work Monday.• Please confirm in writing the date you intend to leave. 