Abu al-Zubayr (may Allah have mercy on him) said that he heard Tawus (may Allah have mercy on him) say: We asked Ibn Abbas (may Allah be pleased with them both) about sitting on the heels, and he said: "It is Sunnah." We said: We regard it as uncouth when a man does it. Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: "Rather, it is the Sunnah of your Prophet Muhammad (peace and blessings be upon him)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2732