Musannaf Ibn Abi Shayba 3:5408

Waki’ narrated to us, he said: Hisham al-Dastawa’i narrated to us, from al-Qasim ibn ‘Awf al-Shaybani, from Zayd ibn Arqam, who said: The Messenger of Allah (may Allah bless him and grant him peace) went out to the people of Quba’ while they were performing the Duha prayer. He said: "The prayer of the oft-repentant (al-awwabin) is when the young camels feel the heat of the forenoon."