Nafi' said, "Ibn 'Umar used to strike his son for making grammatical mistakes."  

عَنْ نَافِعٍ قَالَ كَانَ ابْنُ عُمَرَ يَضْرِبُ وَلَدَهُ عَلَى اللَّحْنِ  

'Abdu'r-Rahman ibn 'Ajlan said, "'Umar ibn al-Khattab, may Allah be pleased with him, passed by two men who were shooting. One man said to another, 'Do hit it (using the letter seen instead of saad).' 'Umar observed, 'A bad grammatical mistake is worse than a bad shot.'"  

فَقَالَ عُمَرُ سُوءُ اللَّحْنِ أَشَدُّ مِنْ سُوءِ الرَّمْيِ