Mbh 3.150.2

Vaisampayana said, “Hearing those words of that high-souled one, Bhimasena with a cheerful heart answered Hanuman, saying, ‘O foremost of monkeys, I take all this as already performed by thee. Good happen to thee. O mighty-armed one! I ask of thee this,--be thou well pleased with me. O powerful one, on thy having become our protector, the Pandavas have found help. Even by thy prowess shall we conquer all foes.” Thus addressed, Hanuman said unto Bhimasena, ‘From fraternal feeling and affection, I will do good unto thee, by diving into the army of thy foes copiously furnished with arrows and javelins. And, O highly powerful one, O hero, when thou shall give leonine roars, then shall I with my own, add force to shouts. Remaining on the flagstaff of Arjuna’s car will I emit fierce shouts that will damp the energy of thy foes. Thereby ye will slay them easily.’ Having said this unto Pandu’s son, and also pointed him out the way. Hanuman vanished at that spot.”