Valmiki Ramayana Aranya Kanda Sarga 43 Shloka 40
Original Shlokaपुरस्तादिह वातापिः परिभूय तपस्विनः। उदरस्थो द्विजान्हन्तिस्वगर्भोऽश्वतरीमिव।।3.43.40।।
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पुरस्तात् earlier, इह here, वातापिः Vatapi, तपस्विनः ascetics, परिभूय defeating, उदरस्थः stationed in the stomach, अश्वतरीम् female mule, स्वगर्भः इव as its own womb, द्विजान् brahmins, हन्ति killed.
Earlier in this forest, Vatapi used to humiliate the ascetic brahmins and to kill them by entering into their stomach like a womb killing the female mule.