Tawus (may Allah have mercy on him) said: I asked Sayyiduna Ibn Abbas (may Allah be pleased with them both) about iq'a' (sitting on both feet), and he said: "It is the Sunnah." I said: We consider it a hardship (an uncomfortable thing) for a man. He said: "It is the Sunnah of your Prophet (peace and blessings be upon him)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10952