Valmiki Ramayana Sundara Kanda Sarga 4 Shloka 2

कपिराजहितंकरः one who wishes the welfare of the king of monkeys, सः he, लङ्कां नगरीम् Lanka city, प्रविश्य entered, अथ then, शत्रूणाम् of enemy, मूर्धनि forehead, सव्यं पादम् left foot, चक्रे placed.

Hanuman, a devotee of the king of monkeys entered the city of Lanka placing his left foot first on the forehead of the city and proceeded (by tradition one enters the enemy's dwelling by placing the left foot first to cause defeat of the enemy).