Aishah (may Allah be pleased with her) said that the Messenger of Allah (peace and blessings be upon him) used to pray four rak'ahs of Duha (forenoon), and would add as much as he wished.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4899
Mu'adhah al-Adawiyyah (may Allah have mercy on her) said: I asked Aishah (may Allah be pleased with her): Did the Messenger of Allah (peace and blessings be upon him) pray the Duha prayer? She said: Yes, four rak'ahs, and he would add as much as Allah willed.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4900
Sayyiduna Nu'aym bin Hammar al-Ghatafani (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him) that Allah Almighty says: "O son of Adam, pray four rak'ahs for Me at the beginning of the day, and I will suffice you until its end."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4901