Mujahid (may Allah have mercy on him) said that Abdullah bin Umar and Ibn Abbas (may Allah be pleased with them both) used to sit on their heels in prayer. Abu al-Zubayr (may Allah have mercy on him) said that Tawus (may Allah have mercy on him) also used to sit on his heels.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2736