صَحِيحُ مُسْلِمٍ
Sahih Muslim 719 d
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
وَحَدَّثَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ، وَابْنُ، بَشَّارٍ جَمِيعًا عَنْ مُعَاذِ بْنِ هِشَامٍ، قَالَ حَدَّثَنِي أَبِي، عَنْ قَتَادَةَ، بِهَذَا الإِسْنَادِ مِثْلَهُ .
A hadith like this has been narrated by Qatada with the same chain of transmitters.
Sahih Muslim, 719 dSahihصحيح