Dhamma-anu-dhamma: practicing Dhamma in accordance to Dhamma

(comprehensive collection of every occurrence of that term will be collected here)
But these 4 suttas in SN 22 are a pretty accurate representation of what that phrase means

SN 22 (derived from b.sujato)

39. In Line With the Teachings

39. Anudhammasutta	39. In Line With the Teachings
Sāvatthinidānaṃ.	At Sāvatthī.
“Dhammānudhammappaṭipannassa, bhikkhave, bhikkhuno ayamanudhammo hoti	“monks, when a monk is practicing in line with the teachings, this is what’s in line with the teachings.
yaṃ rūpe nibbidābahulo vihareyya, vedanāya nibbidābahulo vihareyya, saññāya nibbidābahulo vihareyya, saṅkhāresu nibbidābahulo vihareyya, viññāṇe nibbidābahulo vihareyya.	They should live full of disenchantment for form, feeling, perception, co-doings, and consciousness.
Yo rūpe nibbidābahulo viharanto, vedanāya … saññāya … saṅkhāresu nibbidābahulo viharanto, viññāṇe nibbidābahulo viharanto rūpaṃ parijānāti, vedanaṃ … saññaṃ … saṅkhāre … viññāṇaṃ parijānāti,	Living in this way, they completely understand form, feeling, perception, co-doings, and consciousness.
so rūpaṃ parijānaṃ, vedanaṃ … saññaṃ … saṅkhāre … viññāṇaṃ parijānaṃ parimuccati rūpamhā, parimuccati vedanāya, parimuccati saññāya, parimuccati saṅkhārehi, parimuccati viññāṇamhā,	Completely understanding form, feeling, perception, co-doings, and consciousness, they’re freed from these things.
parimuccati jātiyā jarāmaraṇena sokehi paridevehi dukkhehi domanassehi upāyāsehi, parimuccati dukkhasmāti vadāmī”ti.	They’re freed from rebirth, old age, and death, from sorrow, lamentation, pain, sadness, and distress. They’re freed from suffering, I say.”

40. In Line with the Teachings (2nd)

40. Dutiyaanudhammasutta	40. In Line with the Teachings (2nd)
Sāvatthinidānaṃ.	At Sāvatthī.
“Dhammānudhammappaṭipannassa, bhikkhave, bhikkhuno ayamanudhammo hoti yaṃ rūpe aniccānupassī vihareyya … pe …	“monks, when a monk is practicing in line with the teachings, this is what’s in line with the teachings. They should live observing impermanence in form, feeling, perception, co-doings, and consciousness. …
parimuccati dukkhasmāti vadāmī”ti.	They’re freed from suffering, I say.”

41. In Line with the Teachings (3rd)

41. Tatiyaanudhammasutta	41. In Line with the Teachings (3rd)
Sāvatthinidānaṃ.	At Sāvatthī.
“Dhammānudhammappaṭipannassa, bhikkhave, bhikkhuno ayamanudhammo hoti yaṃ rūpe dukkhānupassī vihareyya … pe …	“monks, when a monk is practicing in line with the teachings, this is what’s in line with the teachings. They should live observing suffering in form, feeling, perception, co-doings, and consciousness. …
parimuccati dukkhasmāti vadāmī”ti.	They’re freed from suffering, I say.”

42. In Line with the Teachings (4th)

42. Catutthaanudhammasutta	42. In Line with the Teachings (4th)
Sāvatthinidānaṃ.	At Sāvatthī.
“Dhammānudhammappaṭipannassa, bhikkhave, bhikkhuno ayamanudhammo hoti yaṃ rūpe anattānupassī vihareyya, vedanāya … saññāya … saṅkhāresu … viññāṇe anattānupassī vihareyya.	“monks, when a monk is practicing in line with the teachings, this is what’s in line with the teachings. They should live observing not-self in form, feeling, perception, co-doings, and consciousness. …
Yo rūpe anattānupassī viharanto … pe … rūpaṃ parijānāti, vedanaṃ … saññaṃ … saṅkhāre … viññāṇaṃ parijānāti, so rūpaṃ parijānaṃ, vedanaṃ … saññaṃ … saṅkhāre … viññāṇaṃ parijānaṃ parimuccati rūpamhā, parimuccati vedanāya, parimuccati saññāya, parimuccati saṅkhārehi, parimuccati viññāṇamhā, parimuccati jātiyā jarāmaraṇena sokehi paridevehi dukkhehi domanassehi upāyāsehi, parimuccati dukkhasmāti vadāmī”ti. Dasamaṃ.	They’re freed from suffering, I say.”
MN 91 “brahmāyu, bhante, brāhmaṇo kālaṅkato. “Sir, Brahmāyu has passed away. Tassa kā gati, ko abhisamparāyo”ti? Where has he been reborn in his next life?” “Paṇḍito, bhikkhave, brahmāyu brāhmaṇo paccapādi dhammassānudhammaṃ, na ca maṃ dhammādhikaraṇaṃ vihesesi. “Mendicants, the brahmin Brahmāyu was astute. He practiced in line with the teachings, and did not trouble me about the teachings. Brahmāyu, bhikkhave, brāhmaṇo pañcannaṃ orambhāgiyānaṃ saṃyojanānaṃ parikkhayā opapātiko hoti, tattha parinibbāyī, anāvattidhammo tasmā lokā”ti. With the ending of the five lower fetters, he’s been reborn spontaneously and will become extinguished there, not liable to return from that world.”