Valmiki Ramayana Ayodhya Kanda Sarga 99 Shloka 12

वने in the forest, तापसाः ascetics, सततम् always, यमेव only whom, आधातुम् to maintain, इच्छन्ति wishing, तस्य that, कृष्णवर्त्मनः of blacktrailed fires, असौ this, सङ्कुलः thick, धूमः smoke, दृश्यते is seen.

Here is the thick smoke emanating from the blacktrailed fire maintained perpetually by the ascetics living in the forest.