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
(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
Ibn Ishaq narrated a hadith about the Messenger of Allah (peace and blessings be upon him) keeping his heels and his toes upright between the two prostrations. Abdullah bin Abi Najih narrates from Mujahid bin Jabr that he heard Abdullah bin Abbas (may Allah be pleased with them both) mention this, and he said to him: "O Abu al-Abbas! By Allah, we used to count this as uncouth on the part of whoever did it." He said: "It is Sunnah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2734
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
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
Abu Zuhayr Mu'awiyah bin Hudayj (may Allah have mercy on him) said: I saw Tawus (may Allah have mercy on him) sitting on his heels, and I said to him: You sit on your heels? He replied: The way you saw me is the very manner of the prayer; I saw the three Abdullahs (Abdullah bin Abbas, Abdullah bin Umar and Abdullah bin al-Zubayr) (may Allah be pleased with them) doing it this way.
Abu Zuhayr said: And I did see him sitting on his heels.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2737
(a) It is narrated from Tawus (may Allah have mercy on him): I saw Sayyiduna Abdullah bin Umar and Ibn Abbas (may Allah be pleased with them both); both of them, in prayer, in the sitting between the two prostrations, would sit on their toes.
(b) Ibrahim (may Allah have mercy on him) said: I asked Ata (may Allah have mercy on him) about this, and he said: Whichever of these ways you adopt will suffice you. If you wish, sit on the tips of your toes, and if you wish, sit on your buttocks.
(c) This is the permitted or Sunnah iq'a' (sitting on the heels) about which we have mentioned the narrations from Ibn Abbas and Ibn Umar (may Allah be pleased with them both). What is meant by it is that a person places the tips of his toes on the ground, places his buttocks on his heels, and his knees are on the ground.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2738