Sahih Bukhari Volume 5, Book 59, Hadith Number 587.

Narrated By Ibn Laila : None informed us that he saw the Prophet offering the Duha (i.e. forenoon) prayer, except Um Ham who mentioned that the Prophet took a bath in her house on the day of the Conquest (of Mecca) and then offered an eight Rakat prayer. She added, “I never saw the Prophet offering a lighter prayer than that prayer, but he was performing perfect bowing and prostrations.”