הלכה על סוכה 108:11

I have written the laws of the commandment about the matter of the additional sacrifices and and the whole topic - as is my custom - regarding the addition of Pesach in the Order of Emor el HaKohanim (Sefer HaChinukh 299). And I will further inform you here that [when] Rosh Chodesh falls on Shabbat, the song of the addition of Rosh Chodesh pushes off the song of the addition of Shabbat, in order to publicize that today is Rosh Chodesh (Sukkah 54b). And it is well-known that the song of the additions of Shabbat in the Temple was the song of Haazinu. And they would divide it into six parts on six Shabbats. And the sign [to remember this] is hay, zayin, yod, vav, lamed, reish (the first letters of each of the parts), in the way that we read it in the synagogue.