Yahya related to me from Malik from Sadaqa ibn Yasar that al-Mughira ibn Hakim saw Abdullah ibn Umar sit back from the two sajdasof the prayer onto the top of his feet. When he had finished, al-Mughira mentioned it to him, and Abdullah ibn Umar explained, "It isnot a sunna of the prayer. I do it because I am ill."
App reference: Book 3, Hadith 53
USC-MSA web (English) reference: Book 3, Hadith 53
Arabic reference: Book 3, Hadith 200