Valmiki Ramayana Bala Kanda Sarga 23 Shloka 6
Original Shlokaतत्राश्रमपदं पुण्यमृषीणामग्य्रतेजसाम् । बहुवर्षसहस्राणि तप्यतां परमं तप:।।1.23.6।।
1 source checked for this shloka
तत्र there, बहुवर्षसहस्राणि for many thousands of years, परमम् utmost, तप: austerities, तप्यताम् while performing austerities, अग्य्रतेजसाम् sages of topmost vital power, ऋषीणाम् of the anchorites, पुण्यम् sacred, आश्रमपदम् hermitage.
There they saw a sacred hermitage of anchorites of great vitality performing severe austerities for thousands of years.