Al-Mughirah bin Hakim (may Allah have mercy on him) said that he saw Abdullah bin Umar (may Allah be pleased with them both) praying; in the prayer, after the two prostrations, he would rise straight up on his feet. When he finished his prayer with the salam, I mentioned this (sitting for rest) to him, and he said: This is not a Sunnah of the prayer; I only do it because I have an ailment.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2762