Sahih Muslim Book - 6, Hadith - 1662

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- يُصَلِّى سُبْحَةَ الضُّحَى قَطُّ. وَإِنِّى لأُسَبِّحُهَا وَإِنْ كَانَ رَسُولُ اللَّهِ -صلى الله عليه وسلم- لَيَدَعُ الْعَمَلَ وَهُوَ يُحِبُّ أَنْ يَعْمَلَ بِهِ خَشْيَةَ أَنْ يَعْمَلَ بِهِ النَّاسُ فَيُفْرَضَ عَلَيْهِمْ.

It was narrated that 'Aishah said: "I never saw the Messenger of Allah (s.a.w) praying the voluntary prayer of Duha but I used to do it. If the Messenger of Allah (s.a.w) gave up an action that he liked to do, it was for fear that the people would do it and it would be made obligatory for them."