Ash-Shama'il Al-Muhammadiyah, Book 40, Hadith 276 (English) ⓘ

'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".

حَدَّثَنَا زِيَادُ بْنُ أَيُّوبَ الْبَغْدَادِيُّ، قَالَ‏:‏ حَدَّثَنَا مُحَمَّدُ بْنُ رَبِيعَةَ، عَنْ فُضَيْلِ بْنِ مَرْزُوقٍ، عَنْ عَطِيَّةَ، عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ، قَالَ‏:‏ كَانَ النَّبِيُّ صلى الله عليه وسلم، يُصَلِّي الضُّحَى حَتَّى نَقُولَ‏:‏ لا يَدَعُهَا، وَيَدَعُهَا حَتَّى نَقُولَ‏:‏ لا يُصَلِّيهَا‏.‏