Vedah Smritah Sadaachaarah Svasyacha priyamaatmanah,
Etad chaturvidham praahuh saakshaat Dharmasya lakshanam
The four corner-stones of Dharma are Veda, Smriti, Sadaachaara and above all, one's own Inner Conscience.
The above Sanskrit śloka can be found at the end of this book (Dharma Bindu - Essence of Dharma Shastra).
What is the source of this śloka? Which scripture contains this?
yaś caturguṇasaṃpannaṃ dharmaṃ veda sa dharmavit
aher iva hi dharmasya padaṃ duḥkhaṃ gaveṣitum [MB, Shanti Parva, 12-130.019a]
Above, Bhishma seems to make an indirect reference to it in the Mahabharata and that reference has been used in this answer.