Reference for Pesachim 234:21
רביעי גומר עליו את ההלל ואומר עליו ברכת השיר:
Then they did not silence him.<span class="x" onmousemove="('comment',' Omitted in MS. M. var. lec. add: And the law is as the elders of Pumbeditha.');"><sup>26</sup></span> <big><b>MISHNAH: </b></big>THEY FILLED THE THIRD CUP FOR HIM. HE THEN RECITES GRACE AFTER MEALS. OVER THE FOURTH [CUP] HE CONCLUDES THE HALLEL, AND RECITES THE GRACE OF SONG.<span class="x" onmousemove="('comment',' The phrase is explained in the GEMARA:');"><sup>27</sup></span> BETWEEN THESE CUPS<span class="x" onmousemove="('comment',' Viz., first, second and third.');"><sup>28</sup></span> HE MAY DRINK IF HE WISHES; BETWEEN THE THIRD AND THE FOURTH HE MAY NOT DRINK. <big><b>GEMARA: </b></big>R'Hanan said to Raba: This proves that Grace after meals requires a cup [of wine]. Said he to him: Our Rabbis instituted four cups as symbolizing freedom:<span class="x" onmousemove="('comment',' This is omitted in Rashbam.');"><sup>29</sup></span> let us perform a religious act with each.<span class="x" onmousemove="('comment',' Hence Grace is recited over the third. But on other occasions a cup may not be required for Grace after meals.');"><sup>30</sup></span> OVER THE FOURTH [CUP] HE CONCLUDES THE HALLEL, AND RECITES THE GRACE OF SONG.