Hadith 502000
Chapter 3: Prayer - كتاب الصلاة
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ صَدَقَةَ بْنِ يَسَارٍ، عَنِ الْمُغِيرَةِ بْنِ حَكِيمٍ، أَنَّهُ رَأَى عَبْدَ اللَّهِ بْنَ عُمَرَ يَرْجِعُ فِي سَجْدَتَيْنِ فِي الصَّلاَةِ عَلَى صُدُورِ قَدَمَيْهِ فَلَمَّا انْصَرَفَ ذَكَرَ لَهُ ذَلِكَ فَقَالَ إِنَّهَا لَيْسَتْ سُنَّةَ الصَّلاَةِ وَإِنَّمَا أَفْعَلُ هَذَا مِنْ أَجْلِ أَنِّي أَشْتَكِي .
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."
Reference Muwatta Malik 502000
USC-MSA web (English) reference : Book 3, Hadith 53