סימן כא סעיף ד
אַךְ בְּיוֹם שֶׁיֵּשׁ בּוֹ מוּסָף, אִם לֹא הִתְפַּלֵּל שַׁחֲרִית, יָכוֹל לְהַשְׁלִימָהּ לְאַחַר מִנְחָה, (דְּכֵיוָן דִּזְמַן תְּפִלַּת מוּסָף, בְּדִיעֲבַד עַד הַלַּיְלָה, כְּמוֹ מִנְחָה אִם כֵּן לֹא עָבַר שְׁתֵּי זְמַנֵּי תְּפִלּוֹת) וּלְאַחַר תְּפִלַּת מוּסָף, אֵינוֹ יָכוֹל לְהַשְׁלִים תְּפִלַּת שַחֲרִית, (דְּכֵיוָן שֶׁמַּזְכִּיר בָּהּ קָרְבַּן מוּסָף, אֵינָהּ רְאוּיָה שֶׁתַּעֲלֶה לִתְפִלַּת שַׁחֲרִית. פְּרִי מְגָדִים סִימָן ק"ח, אַבְרָהָם סָעִיף קָטָן ו'). אַךְ עִם עֲדַיִן לֹא עָבַר זְמַן תְּפִלַּת שַׁחֲרִית, יָכוֹל לְהִתְפַּלֵּל שַׁחֲרִית, אַף עַל פִּי שֶׁכְּבָר הִתְפַּלֵּל מוּסָף.
However, on a day when there is an additional Musaf prayer,5Musaf—an additional tefillah corresponding to special sacrifices that were brought on festive days; e.g. Shabbos, Rosh Chodesh, Yom Tov etc. if you omitted Shacharis [unintentionally], you may compensate for it after the Minchah prayer,6Despite having already prayed two tefillos—Musaf and Minchah. (because the period for Musaf extends, as a last resort, until nightfall as does [the period] for Minchah; as a result, two periods of tefillah have not passed).7Although two tefillos have actually taken place since Shacharis, nevertheless, only one period of prayer has passed, since the times allotted for Musaf and Minchah are identical—from midday until nightfall. But, after the Musaf prayer you may not compensate for [the omitted] Shacharis, (because having already mentioned the Musaf sacrifice, it is inappropriate as a make up for the Shacharis prayer).8Since, as mentioned in the preface to this chapter, two Musaf Shemoneh Esreis would be required, the compensatory Shemoneh Esrei, with the focal point on the special additional sacrifices, is not fitting as compensation for Shacharis. But, if the Shacharis period has not yet passed, you may still pray the Shacharis prayer despite already having prayed Musaf.
תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)