סימן קיג סעיף ו

הַבְּכוֹרִים בֵּין בְּכוֹר לְאָב בֵּין בְּכוֹר לְאֵם, מִתְעַנִּים בְּעֶרֶב פֶּסַח, אֲפִלּוּ חָל בְּעֶרֶב שַׁבָּת. וְגַם הַבָּא אַחַר הַנְּפָלִים, צָרִיךְ לְהִתְעַנּוֹת (עַיֵן דָּגוּל מֵרְבָבָה). וְכַל זְמַן שֶׁהַבְּכוֹר קָטָן, הָאָב מִתְעַנֶּה תַּחְתָּיו. בִּסְעוּדַּת מִצְוָה אִם מֻתָּרִים לֶאֱכוֹל, תַּלְיָא בְּמִנְהַג הַמְּקוֹמוֹת.

The firstborn sons either of the father or of the mother must fast on erev Pesach,⁶However, if you had a severe headache or your eyes hurt you, you need not complete the fast. Similarly, if fasting affects you so that after the fast you are unable to eat, except light foods, and it is possible you will be unable to eat matzah, marror and drink four cups of wine at night, it is better not to fast. (Ibid. 470:2) even if it occurs on Friday. A firstborn who was born after a miscarriage must also fast. When the firstborn is a minor, his father fasts instead of him. Whether firstborn are permitted to eat at a mitzvah meal depends on the local custom.⁷Our custom is to permit the first born to eat at a Siyum (the completion) of a Maseches, even though they, themselves, did not study it. (Ibid. 470:10)

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.)