Musannaf Ibn Abi Shayba 28:325

Zayd ibn al-Hubab narrated to us, he said: Musa ibn Ubaydah informed us, from Qays ibn Abd al-Rahman ibn Sa'sa'ah, from Sa'd ibn Ibrahim, from Ali, from his grandfather Abd al-Rahman ibn Awf, who said: I reached the Prophet, may Allah bless him and grant him peace, while he was prostrating. When he finished, I said: "O Messenger of Allah, you prolonged the prostration." He said: "I prostrated in gratitude to Allah for what He tested me with regarding my nation."