Al-Matalib al-'Aliyah 666 — Forgetfulness in prayer

It is narrated from Humayd bin Tarkhan that Abdullah bin Shaqiq led us in the Asr prayer, and he made us perform two prostrations (of forgetfulness, at the end of the prayer), although we had not noticed any forgetfulness or mistake in his prayer. When he gave the salam, the people mentioned this to him and said: "We did not notice any forgetfulness or mistake (in your prayer) (so why did you make these prostrations)?" He replied: "In fact, I was talking to myself (during the prayer) (i.e. my attention had wandered, so I made these prostrations as a precaution)."
The chain of this report is sahih (authentic), and it describes the practice of an eminent Successor regarding performing the prostration of forgetfulness when whisperings or thoughts arise during the prayer.