Reference for Sanhedrin 97:12
(שמואל ב ג, כז) ויטהו יואב אל תוך השער לדבר אתו בשלי אמר רבי יוחנן שדנו דין סנהדרי א"ל מ"ט קטלתיה לעשאל עשאל רודף היה היה לך להצילו באחד מאיבריו לא יכילי ליה השתא בדופן חמישית כוונת ליה באחד מאיבריו לא יכלת ליה
Thus he asked him: 'Whydidst thou kill Asahel?' — 'Because Asahel was my pursuer.' 'Then thou shouldsthave saved thyself<span class="x" onmousemove="('comment',' Lit., 'him', i.e., save the pursuer from committing a crime, v. supra p. 326, n 8. ');"><sup>32</sup></span> at the cost ofone of his limbs!' 'I could not do that,' [he answered]. 'If thou couldstaim exactly at his fifth rib, couldst thou not have prevailed against himby [wounding] one of his limbs?' 'To speak with him ba-sheli [quietly]:' Rab Judah said in Rab's name: [Hespoke to him] concerning the putting off [of theshoe].<span class="x" onmousemove="('comment',' The word [H] is here derived from [H] to draw or pull off. Joab is supposed to have inquired from Abner in what way a one-armed woman would loosen the shoe in the ceremony of halizah (v. Deut. XXV, 9). On his replying that she would do it with her teeth (cf. Yeb 105a), he asked him to demonstrate it, and as he stooped low to do so, he smote him. This incident is hinted at in David's words of farewell to Solomon: He (sc. Joab) shed the blood of war in peace, — and put the blood of war in the shoes that were on his feet (I Kings II, 5). ');"><sup>33</sup></span>