Maliks Muwatta Book 3, Hadith Number 53.

Yahya related to me from Malik from Sadaqa ibn Yasar that al-Mughira ibn Hakim saw Abdullah ibn Umar sit back from the two sajdas of the prayer onto the top of his feet. When he had finished, al-Mughira mentioned it to him, and Abdullah ibn Umar explained, “It is not a sunna of the prayer. I do it because I am ill.”