What is the source of the Sanskrit śloka that lists the four corner-stones of dharma?

Question

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.

Answer 1

This Shloka is available in Manu Smriti, Chapter 2, Verse 12.

वेदः स्मृतिः सदाचारः स्वस्य च प्रियमात्मनः ।

एतच्चतुर्विधं प्राहुः साक्षाद् धर्मस्य लक्षणम् ॥ १२ ॥

vedaḥ smṛtiḥ sadācāraḥ svasya ca priyamātmanaḥ |

etaccaturvidhaṃ prāhuḥ sākṣād dharmasya lakṣaṇam || 12 ||

The Veda, the Smṛti, the Practice of cultured Men, and what is agreeable to oneself—these directly constitute the fourfold means of knowing Dharma.—(12)

Answer 2

This Shloka is also found in at-least two Puranas, besides other scriptures.

1. Bhavishya Purana:

• Verse 57, Chapter 7, Part 1 (Brahma Khanda).

Chapter 7 - Brahma Parva, Bhavishya Purana

वेदः स्मृतिः सदाचारः स्वस्य च प्रियमात्मनः ।
एतच्चतुर्विधं विप्राः साक्षाद्धर्मस्य लक्षणम् ॥ ५७ ॥

Meaning:

Vedas, smritis, suitable (approved) tradition, and what is agreeable to one's conscience, the wise have declared to be the four direct pieces of evidence of dharma.

This verse is exactly similar to that asked in the question

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2. Vishnu-Dharmottara Purana

• Verse 63, Chapter 233, Part 3

Chapter 233, Tritiya Khanda, Vishnudharmottara Purana

वेदः स्मृतिः सदाचारः स्वस्य च प्रियमात्मनः ।
एतच्चतुर्विधं प्राहुः साधूनां धर्मलक्षणम् ॥ ६३ ॥

Here, the second line of the verse is a bit different from the original one. Instead of साक्षाद्धर्मस्य लक्षणम् we have साधूनां धर्मलक्षणम्

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On a side Note:

As already said by one of the users in the comments, a similar verse is also found in the Yajnavalkya Smriti

Verse 7, Chapter 1, Yajnavalkya Smriti

श्रुतिः स्मृतिः सदाचारः स्खस्य च प्रियमात्मनः ।
सम्यक्संकल्पजः कामो धर्ममूलमिदं स्मृतम् ॥ ७ ॥

VII. — The Sruti, the Smiriti, the conduct of good men, what appears pleasant to one's own self, and the desire that springs from a good resolution, is said to be the root of Dharma.

^{One may access the English Translation along with the commentary of Vijnanevara called the Mitaksara from here}