(a) It is narrated from Mus'ab bin Sa'd (may Allah have mercy on him) that he said: I prayed standing beside my father (may Allah be pleased with him). I placed my hands between my knees, and he pinched my hand. Mus'ab (may Allah have mercy on him) said: I prayed beside him once again and placed my hands between my thighs. He pinched my hand again and said: "My dear son! We too used to do this before; then we were commanded to place the palms on the knees."
(b) Imam Muslim (may Allah have mercy on him) recorded it in his Sahih from Qutaybah and others, and in it there is the addition: "We were forbidden from this and were commanded to place the hands on the knees."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2544