Sahih Muslim 718
Sahih Muslim · Book 6: The Book of Prayer - Travellers · It is recommended to pray Duha, the least of which is two rak`ah, the best of which is eight, and the average of which is four or six, and encouragement to do so regularly
'Urwa reported 'A'isha to be sayidg:
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى ، قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنِ ابْنِ شِهَابٍ ، عَنْ عُرْوَةَ ، عَنْ عَائِشَةَ ، أَنَّهَا قَالَتْ مَا رَأَيْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يُصَلِّي سُبْحَةَ الضُّحَى قَطُّ . وَإِنِّي لأُسَبِّحُهَا وَإِنْ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم لَيَدَعُ الْعَمَلَ وَهُوَ يُحِبُّ أَنْ يَعْمَلَ بِهِ خَشْيَةَ أَنْ يَعْمَلَ بِهِ النَّاسُ فَيُفْرَضَ عَلَيْهِمْ .
I have never seen the Messenger of Allah (ﷺ) observing the supererogatory prayer of the forenoon, but I observed it. And if the Messenger of Allah (ﷺ) abandoned any act which he in fact loved to do, it was out of fear that if the people practised it constantly, it might become obligatory for them.