I was looking in a book of collected ideas of Rav Elchanan (Kovetz Maamarim ve-Iggrot).

There is a one line comment there about daf hayomi:

דף היומי הוא רעיון נהדר, אבל אפ אין חוזרים על הדף הרבה פעמים, הלא בפירוש הגידו חז”ל סנהדרין צ”ט ע”א) שהוא כזורע ואינו קוצר.
“Daf Yomi is a wonderful idea. But if they don’t review the daf many times, then Chazal have told us (Sanhedrin 99a) that it is like planting and not reaping.”

Also, while on the topic of reviewing Shas (Talmud) he writes:

אדם מישראל צריך למצוא פנאי לחזור ש”ס בכל שנה.

“A Jewish person must find an opportunity to review Shas every year.”

Both of those statements are מפי השמועה – they were heard by students.