Reference for Yevamot 204:3

והא תניא בין שנתכוון הוא ולא נתכוונה היא בין שנתכוונה היא ולא נתכוון הוא חליצתה פסולה עד שיתכוונו שניהם כאחד הכי קאמר אע"ג דחזינן אין חוששין שמא כוונו

But, surely, it was taught: Whether he<span class="x" onmousemove="('comment',' The levir. ');">⁷</span> had the intention<span class="x" onmousemove="('comment',' Where halizah was performed. ');">⁸</span> [of performing the commandment of <i>halizah</i>] and she had no such intention, or whether she had such intention and he did not, <i>halizah</i> is invalid, it being necessary<span class="x" onmousemove="('comment',' Lit., 'until'. ');">⁹</span> that both shall at the same time have such intention!<span class="x" onmousemove="('comment',' Tosef. Yeb. XII, infra 106a. Why then should the removal of a sandal as a mere friendly act ever be regarded as a valid halizah? ');">¹⁰</span>

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