Sayyiduna Samurah bin Jundub (may Allah be pleased with him) relates that the Messenger of Allah (peace and blessings be upon him) forbade iq'a' in prayer (that is, sitting like a beast of prey with the buttocks on the ground and the knees raised). This hadith is sahih on the condition of Imam Bukhari, but the two Shaykhs did not record it, while there is a narration permitting iq'a' (sitting on the heels) that is sahih on the condition of Imam Muslim:

Tawus (may Allah have mercy on him) relates: I asked Sayyiduna Ibn Abbas (may Allah be pleased with them both) about iq'a' (in prayer), that is, sitting on the heels between the two prostrations, and he said: It is Sunnah. I said: We regard it as a kind of harshness or coarseness. Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: (It is not so; rather,) it is the Sunnah.

Sayyiduna Ibn Umar (may Allah be pleased with them both) relates that the Prophet (peace and blessings be upon him) forbade a man who was sitting in prayer leaning on his left hand, and said: "Indeed, this is the prayer (manner) of the Jews." This hadith is sahih on the condition of the two Shaykhs, but they did not record it.