Shama-il Tirmidhi Chapter 040, Hadith Number 005 (276).

'Abu Sa'eed Al-Khudari Radiyallahu 'Anhu relates: "Rasulullah Sallallahu 'Alayhi Wasallam at times performed the salaatut duha with such regularity, that we thought, he would not leave it. At times he left it (fearing it might become fard or because of some advantage), till we thought he left it completely, and would not perform it again".

Commentary
"Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam taking the ummah into consideration, discontinued many devotions, of which there are many examples. Sayyiditina 'Aayeshah Radiyallahu 'Anha says: 'Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam very much liked to do a few things, but did not do it, fearing it would become fard on the ummah'".