Reference for Pesachim 215:11

ת"ש (אמר) אביי כי הוינן בי מר זגינן אבירכי דהדדי כי אתינן לבי רב יוסף אמר לן לא צריכתו מורא רבך כמורא שמים

Lying on the back is not reclining; reclining on the right side is not reclining.<span class="x" onmousemove="('comment',' Since he must eat with his right hand.');">¹²</span> Moreover he may put [his food] into the windpipe before the gullet,<span class="x" onmousemove="('comment',' if he eats lying on his back may go down the wrong way.');">¹³</span>

Jastrow

Jastrow