Reference for Bava Batra 346:8

תניא נמי הכי המלוה את חבירו על ידי ערב לא יתבע ערב תחלה ואם אמר על מנת שאפרע ממי שארצה יתבע ערב תחלה

Thus it was also taught [elsewhere]: If [a man] lends [money] to another on a guarantor's security, [payment] shall not be demanded [from the] guarantor [in the] first instance. If, however, [the creditor] said, 'On condition that I may exact payment from whom I will' the guarantor may be called upon first.<span class="x" onmousemove="('comment',' V. infra. ');">¹⁴</span> Said R. Huna: Whence [may it be deduced] that a guarantor becomes responsible [for a debt he has guaranteed]?<span class="x" onmousemove="('comment',' By his mere verbal undertaking, though it was not attended by a kinyan. ');">¹⁵</span>

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