Sahih Muslim 717
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
'Abdullah b. Shaqiq reported:
وَحَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ مُعَاذٍ ، حَدَّثَنَا أَبِي ، حَدَّثَنَا كَهْمَسُ بْنُ الْحَسَنِ الْقَيْسِيُّ ، عَنْ عَبْدِ اللَّهِ بْنِ شَقِيقٍ ، قَالَ قُلْتُ لِعَائِشَةَ أَكَانَ النَّبِيُّ صلى الله عليه وسلم يُصَلِّي الضُّحَى قَالَتْ لاَ إِلاَّ أَنْ يَجِيءَ مِنْ مَغِيبِهِ
I aksed 'A'isha whether the Messenger of Allah (ﷺ) used to observe the forenoon prayer. She said: No, except when he came back from a journey.