(a) It is narrated from Sayyiduna Sa'd (may Allah be pleased with him) that at first we used to place our hands before our knees; then we were commanded "to place our knees before our hands."
So he narrated it; but what is well known from Mus'ab from his father is the hadith about the abrogation of tatbiq (pressing the palms together between the knees in bowing), and Allah knows best. Abd al-Aziz al-Darawardi has another chain for it, and I consider it nothing but an error.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2637