Reference for Nazir 5:6
הִרְתִּיעַ לַאֲחוֹרָיו, אֵינוֹ נָזִיר. רַבִּי שִׁמְעוֹן אוֹמֵר, יֹאמַר, אִם הָיָה כִדְבָרַי, הֲרֵינִי נְזִיר חוֹבָה. וְאִם לָאו, הֲרֵינִי נְזִיר נְדָבָה:
If he drew back, [If the one coming towards them went back, so that it was not known who it was], he is not a Nazirite. [Not one of them is a Nazirite, for one does not place himself in a position of doubt, it being assumed at the time of the vow that if there is no clear determination the vow does not obtain.] R. Shimon says: He must say: "If it were as I said, I am a Nazirite by obligation; if not, I am a Nazirite by election." [R. Shimon is consistent with his view that in an instance of doubt as to whether one is or is not a Nazirite, the stringent option must be followed. And what is their remedy? (For they cannot bring an offering in doubt.) Each one must stipulate that if it is not as he vowed, he is a Nazirite by election. The halachah is not in accordance with R. Shimon.]