Hazrat Mus'ab bin Sa'd narrates that when I used to perform ruku', I would place both my hands between my knees. My respected father, Sayyiduna Sa'd (may Allah be pleased with him), saw me doing this and forbade me, and said: Indeed, we used to do the same, then we were forbidden from it, then we were commanded to raise them towards our knees (i.e., to place them on the knees).