Musannaf Ibn Abi Shayba 33:119

Abu Khalid al-Ahmar narrated to us, from Hajjaj, from Makhul, who said: It has reached me that the Messenger of Allah (may Allah bless him and grant him peace) said: "No servant devotes his deeds sincerely to Allah for forty mornings except that the springs of wisdom appear from his heart onto his tongue."