Valmiki Ramayana Bala Kanda Sarga 63 Shloka 15

राम O Rama, तस्य for him, उत्तरे पर्वते on the northern mountains, वर्षसहस्रम् a thousand years, घोरम् fearful, तप: penance, उपासत: while performing, देवतानाम् to devatas, भयम् अभूत् fear arose.

O Rama while he was performing rigorous penance for a thousand years on the northern mountains, the gods were gripped with fear. (lest he should exceed them).