(a) Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: It is Sunnah in prayer that the buttocks touch the heels, that is, sitting on the heels is Sunnah.
(b) In the narration of Sufyan there is the addition that sitting in this way between the two prostrations is Sunnah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2733