(a) Abu Hurayrah (may Allah be pleased with him) says: "Ikhtisar (placing the hands on the hips) in prayer has been forbidden."
(b) In another narration of al-Bukhari it is: "about al-khasr in prayer", meaning placing the hand on the flank in prayer.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3561