Brahmanas and Kshatriyas and Vaishyas, these three orders, are considered as twice born. By delivering instruction to these, a Brahmana does not commit any sin.
ब्राह्मणा: क्षत्रिया वैश्यास्त्रयो वर्णा द्विजातयः। एतेषु कथयन् राजन् ब्राह्मणो न प्रदुष्यति॥