Musnad Ahmad 25:994

Muhammad ibn Bakr and Abd al-Razzaq narrated to us, they said: Ibn Jurayj informed us, Abu al-Zubayr informed me that he heard Tawus saying: We said to Ibn Abbas regarding the *iq'a* (sitting on the heels) on the feet? He said: "It is the Sunnah." He said: So we said: "We indeed see it as harshness to the man." Ibn Abbas said: "It is the Sunnah of your Prophet, may Allah bless him and grant him peace."