Musnad Ahmad 8:131

Yahya narrated to us, from Isma'il, from al-Zubayr ibn 'Adi, from Mus'ab ibn Sa'd, who said: I prayed with Sa'd, and I placed my hands like this—and Yahya described the *tatbiq* (clasping the hands together between the knees)—so he struck my hand and said: "We used to do this, then we were commanded to place them upon the knees."