Sahih Bukhari Volume 002, Book 021, Hadith Number 275t.
Narated By Anas bin Sirin : I heard Anas bin Malik al-Ansari saying, "An Ansari man, who was very fat, said to the Prophet, 'I am unable to present myself for the prayer with you.' He prepared a meal for the Prophet and invited him to his house. He washed one side of a mat with water and the Prophet offered two Rakat on it." So and so, the son of so and so, the son of Al-Jarud asked Anas, "Did the Prophet use to offer the Duha prayer?" Anas replied, "I never saw him praying (the Duha prayer) except on that day."