Sahih Muslim 720
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
Abu Dharr reported Allah's Apostle (ﷺ) as saying:
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مُحَمَّدِ بْنِ أَسْمَاءَ الضُّبَعِيُّ ، حَدَّثَنَا مَهْدِيٌّ، - وَهُوَ ابْنُ مَيْمُونٍ - حَدَّثَنَا وَاصِلٌ ، مَوْلَى أَبِي عُيَيْنَةَ عَنْ يَحْيَى بْنِ عُقَيْلٍ ، عَنْ يَحْيَى بْنِ يَعْمَرَ ، عَنْ أَبِي الأَسْوَدِ الدُّؤَلِيِّ ، عَنْ أَبِي ذَرٍّ ، عَنِ النَّبِيِّ صلى الله عليه وسلم أَنَّهُ قَالَ " يُصْبِحُ عَلَى كُلِّ سُلاَمَى مِنْ أَحَدِكُمْ صَدَقَةٌ فَكُلُّ تَسْبِيحَةٍ صَدَقَةٌ وَكُلُّ تَحْمِيدَةٍ صَدَقَةٌ وَكُلُّ تَهْلِيلَةٍ صَدَقَةٌ وَكُلُّ تَكْبِيرَةٍ صَدَقَةٌ وَأَمْرٌ بِالْمَعْرُوفِ صَدَقَةٌ وَنَهْىٌ عَنِ الْمُنْكَرِ صَدَقَةٌ وَيُجْزِئُ مِنْ ذَلِكَ رَكْعَتَانِ يَرْكَعُهُمَا مِنَ الضُّحَى " .
In the morning charity is due from every bone in the body of every one of you. Every utterance of Allah's glorification is an act of charity. Every utterance of praise of Him is an act of charity, every utterance of profession of His Oneness is an act of charity, every utterance of profession of His Greatness is an act of charity, enjoining good is an act of charity, forbidding what is distreputable is an act of charity, and two rak'ahs which one prays in the forenoon will suffice.