Valmiki Ramayana Sundara Kanda Sarga 1 Shloka 159
Original Shlokaप्रविश्य वदनं मेऽद्य गन्तव्यं वानरोत्तम। वर एष पुरा दत्तो मम धात्रेति सत्वरा।।5.1.159।। व्यादाय विपुलं वक्त्रं स्थिता सा मारुतेः पुरः।
1 source checked for this shloka
वानरोत्तम O best of the vanaras, अद्य today, मे my, वदनम् face (mouth), प्रविश्य after entering, गन्तव्यम् you may go, एषः वरः such a boon, पुरा past, मम to me, धात्रा by Brahma, दत्तः is given, इति thus, सत्वरा quickly, विपुलम् wide, वक्त्रम् mouth, व्यादाय opened, सा she, मारुतेः vanara's, पुरः front, स्थिता stood.
"O best vanara you must pass through my mouth today. Such is the boon granted to me by Brahma in the past". Then she quickly opened her big mouth and stood.