مُسْنَدُ أَحْمَدَ
Musnad Ahmad 1570
Musnad Abu Ishaq Sa'd bin Abi Waqqas
It was narrated that Mus"ab bin Sa"d said: I prayed with Sa"d and I did this with my hands - Yahya described putting the two hands together between the knees. He struck my hand and said: We used to do that, then we were instructed to lift our hands to the knees.
Musnad Ahmad 1570Sahih (Darussalam) [, al-Bukhari (790) and Muslim (535)]