It is narrated from Muhammad bin Ajlan that Abu al-Zubayr informed him that he saw Abdullah bin Umar (may Allah be pleased with them both) prostrating during prayer; when he raised his head from the (first) prostration, he would sit on his toes and say: "This is Sunnah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2735