Sahih Muslim , Hadith 718 ⓘ

'Urwa reported 'A'isha to be sayidg: I have never seen the Messenger of Allah observing the supererogatory prayer of the forenoon, but I observed it. And if the Messenger of Allah 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.

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 ‏.‏