Muhammad bin Sirin (may Allah have mercy on him) narrates from Abu Hurayrah (may Allah be pleased with him) that "Ikhtisar in prayer, that is, placing the hands on the hips, has been forbidden." I said to Hisham: Did he narrate it from the Prophet (peace and blessings be upon him)? He indicated with a nod of his head: Yes.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3564