Valmiki Ramayana Sundara Kanda Sarga 63 Shloka 18

अस्य कर्मणः that duty, साधने capacity, हनूमतः Hanuman, अन्यः others, हेतुः the reason being, न हि not indeed, कार्यसिद्धि: capacity to accomplish, मतिश्चैव even wisdom, व्यवसायश्च even strenuous effort, वीर्यं च and virility, श्रुतं चापि enthusiasm, तस्मिन् for him, वानरपुङ्गवे to only the foremost of the monkys, प्रतिष्ठितम् are wellestablished.

"Hanuman alone has the ability to do it, the reason being he has the capacity the wisdom, streneous effort, virility and enthusiasm to accomplish it. These attributes are wellestablished in him.