Gevurot Meyruach Hakodesh 1:21-22

It is necessary, therefore, that one of the anashim who accompanied us during all the time in which Rebbe, Melech HaMoshiach Adoneinu Yehoshua went in and went out among us, Beginning from the tevilah of teshuva of Yochanan until the day when Moshiach was taken up from us — one of these should become Eidus (Witness) with us to the Techiyas HaMoshiach.