Musnad Ahmad 8:137

Waki’ narrated to us, Ibn Abi Khalid narrated to us, from al-Zubayr ibn ‘Adi, from Mus’ab ibn Sa’d, who said: "When I would bow, I used to place my hands between my knees." He said: "My father, Sa’d ibn Malik, saw me and forbade me from it. And he said: 'We used to do that, then we were forbidden from it.'"