Yosef Karo, Translation:Shulchan Aruch — Orach Chaim 26Laws Relating to a Person Who Has Only One Tefillin
“ Laws Relating to a Person Who Has Only One Tefillin דין מי שאין לו אלא תפלה אחת Paragraph 1 — If he only has one tefillin, he should don the one he has and bless upon it, for each tefillin individually is its own mitzvah. And the same law applies if he has two of them, and there is any unforeseen circumstance where he is only able to don one, he should wrap the one that he is able. ”
