This is not part of Ahmad's speech, rather he described it to us as the sitting at the end of the prayer.
• I heard Ishaq say: "The sitting in prayer is to lay the left [foot] flat and keep the right [foot] upright. If he spreads the left [foot] then places the right [foot] upon it and drapes them both, though he strives to keep his right foot upright, this is more beloved to us in his final sitting for obligatory prayers, and likewise for voluntary prayers."
948 - Abu Hisham Muhammad ibn Nasr narrated to us, saying: Hassan ibn Ibrahim said: "I saw Sufyan while we were in Jeddah spreading his legs in prayer, so I said to him: 'I saw you sit in a manner I have not seen you sit before?' He replied: 'It reached us from the Prophet (peace and blessings of Allah be upon him) that he used to sit in that manner.'"
949 - Yahya ibn 'Abd al-Hamid narrated to us, saying: 'Abd al-Wahid ibn Ziyad narrated to us, from 'Uthman ibn Hakim, from 'Amir ibn 'Abd Allah ibn al-Zubayr, from his father, who said: "When the Prophet (peace and blessings of Allah be upon him) sat in prayer, he would place his left leg between his thigh and his calf."
• Harb said: "In my opinion, this is al-Tawarruk."