I came in to see the Messenger of Allah ﷺ while a scribe was sitting before him, and I heard him say, "Place the pen on your ear, for it helps the one dictating to remember."
حدثنا قتيبة حدثنا عبد الله بن الحارث عن عنبسة عن محمد بن زاذان عن أم سعد
دخلت على رسول الله ﷺ وبين يديه كاتب فسمعته يقول ضع القلم على أذنك فإنه أذكر للمملي
No references are available.
[Its chain of transmission is very daif] Ibn Saad reported it, 2/359, from the hadith of Anbasah bin Abdul-Rahman Qurashi with the same chain, and by way of Tirmidhi, Ibn Jawzi mentioned it in Al-Mawduat, 1/259. See hadith 2699 regarding the status of Anbasah and Muhammad bin Zazan. The hadith has two corroborating narrations, both very daif, found with Abu Naeem in Akhbar Asbahan, 2/337, and Daylami and others, with accused narrators in both of their chains.