Musnad Abu Ya'la al-Mawsili 149:10

Shayban ibn Farrukh narrated to us, Tayyib ibn Salman narrated to us, he said: Amrah said: I heard the Mother of the Believers say: The Messenger of Allah, may Allah bless him and grant him peace, used to pray the Duha prayer as four rak'ahs, not separating between them with speech.