SB 9.15.10

tad viditvā muniḥ prāha
 patnīṁ kaṣṭam akāraṣīḥ
ghoro daṇḍa-dharaḥ putro
 bhrātā te brahma-vittamaḥ
Word for word: 
tat — this fact; viditvā — having learned; muniḥ — the great sage; prāha — said; patnīm — unto his wife; kaṣṭam — very regrettable; akāraṣīḥ — you have done; ghoraḥ — fierce; daṇḍa-dharaḥ — a great personality who can punish others; putraḥ — such a son; bhrātā — brother; te — your; brahma-vittamaḥ — a learned scholar in spiritual science.
Translation: 
When the great sage Ṛcīka returned home after bathing and understood what had happened in his absence, he said to his wife, Satyavatī, “You have done a great wrong. Your son will be a fierce kṣatriya, able to punish everyone, and your brother will be a learned scholar in spiritual science.”
Purport: 

A brāhmaṇa is highly qualified when he can control his senses and mind, when he is a learned scholar in spiritual science and when he is tolerant and forgiving. A kṣatriya, however, is highly qualified when he is fierce in giving punishment to wrongdoers. These qualities are stated in Bhagavad-gītā (18.42-43). Because Satyavatī, instead of eating her own oblation, had eaten that which was meant for her mother, she would give birth to a son imbued with the kṣatriya spirit. This was undesirable. The son of a brāhmaṇa is generally expected to become a brāhmaṇa, but if such a son becomes fierce like a kṣatriya, he is designated according to the description of the four varṇas in Bhagavad-gītā (cātur-varṇyaṁ mayā sṛṣṭaṁ guṇa-karma-vibhāgaśaḥ). If the son of a brāhmaṇa does not become like a brāhmaṇa, he may be called a kṣatriya, vaiśya or śūdra, according to his qualifications. The basic principle for dividing society is not a person’s birth but his qualities and actions.