I's use evalc to capture the symbolic expression as a string and then write it out:
% Something symbolic
syms x
y = [x sin(x) cos(x)].'*[x sin(x) cos(x)] % some symbolic matrix
%String it
str = evalc('y'); % to string
% Write it
fid = fopen('out.txt','w');
fprintf(fid,str);
fclose(fid);
