Midrash for Bava Batra 253:8

אמר רבא תניא כוותיה דר' אמי בן ולא טומטום בכור ולא ספק

Raba said: It was taught in agreement with [the view] of R. Ammi:<span class="x" onmousemove="('comment',' That a tumtum, though found after an operation to be male, is not entitled to the birthright. ');">²⁵</span> [The expression.] a Son,<span class="x" onmousemove="('comment',' Deut. XXI. is. ');">²⁶</span> [Implies], but not a <i>tumtum</i>;<span class="x" onmousemove="('comment',' I.e., a birth, though found later to be made. ');">²⁷</span> [the expression] a firstborn,<span class="x" onmousemove="('comment',' Ibid. ');">²⁸</span>

Sifrei Devarim