אורח חיים תכ״דOrach Chayim 424

Se’if 1א׳

מַזְכִּירִין יַעֲלֶה וְיָבֹא בְּבִרְכַּת הַמָּזוֹן, וְאִם לֹא אָמַר אֵין מַחֲזִירִין אוֹתוֹ. וְאִם נִזְכַּר קֹדֶם שֶׁהִתְחִיל הַטּוֹב וְהַמֵּטִיב, אוֹמֵר: בָּרוּךְ שֶׁנָּתַן רָאשֵׁי חֳדָשִׁים לְעַמּוֹ יִשְׂרָאֵל לְזִכָּרוֹן. הַגָּה: וְעַיֵּן לְעֵיל סִימָן קפ"ח סָעִיף ז'.

The law of bringing up "Ya'aleh v-Yavo" in the grace after meals, which has one clause: We bring up "Ya'aleh v-Yavo in the grace after meals. And if it was not said, it is not necessary to repeat it. And if it is remembered before one started the fourth blessing, say, "Blessed be the one who gave Rosh Ḥodesh to His people Israel as a reminder." (Rema: See above, SA Orach Chayim 188:7.)