Valmiki Ramayana Sundara Kanda Sarga 47 Shloka 29

अयम् he, न उपेक्षितः not disregard, नाभिभवेत् he will not overtake me, न खलु indeed, रणे in battle, अस्य his, पराक्रमः valour, वर्धते हि is increasing, अद्य now, प्रमापणं त्वेव killing him only, मम for me, रोचते is proper, वर्धमानः growing, अग्निः fire, उपेक्षितुम् to neglect, न क्षमः not proper

If I ignore him now, he would get the better of me (I have to consider his challenge seriously). His valour in the battle is growing. It is proper to subdue him now. A spreading fire cannot be neglected.