Shiva Purana
आस्तिकश्चेत्प्रमादाद्यैः सदाचारादविच्युतः ॥ न दुष्यति नरो नित्यं तस्मादास्तिकतां व्रजेत् ॥ ५९ ॥
If he is a believer he will not err from the path of good conduct or commit errors or get defiled. Hence one shall be a believer in God and the Vedas.
english translation
AstikazcetpramAdAdyaiH sadAcArAdavicyutaH ॥ na duSyati naro nityaM tasmAdAstikatAM vrajet ॥ 59 ॥
hk transliteration by Sanscript