Talmud for Nazir 2:9
הֲרֵינִי נָזִיר, וְנָזִיר כְּשֶׁיִּהְיֶה לִי בֵן, הִתְחִיל מוֹנֶה אֶת שֶׁלּוֹ וְאַחַר כָּךְ נוֹלַד לוֹ בֵן, מַשְׁלִים אֶת שֶׁלּוֹ וְאַחַר כָּךְ מוֹנֶה אֶת שֶׁל בְּנוֹ. הֲרֵינִי נָזִיר כְּשֶׁיִּהְיֶה לִי בֵן וְנָזִיר, הִתְחִיל מוֹנֶה אֶת שֶׁלּוֹ וְאַחַר כָּךְ נוֹלַד לוֹ בֵן, מַנִּיחַ אֶת שֶׁלּוֹ וּמוֹנֶה אֶת שֶׁל בְּנוֹ, וְאַחַר כָּךְ מַשְׁלִים אֶת שֶׁלּוֹ:
(If one said:) "I shall be a Nazirite," and ("I shall be) a Nazirite when I have a son," [taking upon himself Naziritism, unqualified, and another Naziritism when he has a son] — if he started counting his, and then a son were born to him, he completes his [first, and brings an offering], and then counts for that of his son. (If he said:) "I shall be a Nazirite when I have a son," and ("I shall be) a Nazirite," [taking upon himself first the Naziritism for his son, and beginning to count his own, after which a son were born to him, before the thirty days were completed], he suspends his own, counts for his son, and then completes his own. [For since he first took upon himself Naziritism for his son, as soon as he is born he must suspend his own and count for his son and then complete his own.]