Valmiki Ramayana Bala Kanda Sarga 29 Shloka 3

राम O Rama, एष: this place, महात्मन: of the glorious, वामनस्य Vamana's, पूर्वाश्रम: formerly hermitage of, सिद्धाश्रम: Siddhashrama, इति thus, ख्यात: known, हि indeed, महातपा: great ascetic, अत्र here, सिद्ध: succeeded in his ascetic pratices.

"O Rama this was the hermitage of glorious Vamana. The great sage her performed his ascetic pratices here, hence it is known as siddhashrama.