Sahih Bukhari Volume 5, Book 62, Hadith 3738

وَقَالَ نُعَيْمٌ عَنِ ابْنِ الْمُبَارَكِ، أَخْبَرَنَا مَعْمَرٌ، عَنِ الزُّهْرِيِّ، أَخْبَرَنِي مَوْلًى، لأُسَامَةَ بْنِ زَيْدٍ‏.‏ أَنَّ الْحَجَّاجَ بْنَ أَيْمَنَ ابْنِ أُمِّ أَيْمَنَ،، وَكَانَ، أَيْمَنُ ابْنُ أُمِّ أَيْمَنَ أَخَا أُسَامَةَ لأُمِّهِ، وَهْوَ رَجُلٌ مِنَ الأَنْصَارِ، فَرَآهُ ابْنُ عُمَرَ لَمْ يُتِمَّ رُكُوعَهُ وَلاَ سُجُودَهُ فَقَالَ أَعِدْ‏.‏

The freed slave of Usama bin Zaid said, "Al-Hajjaj bin Aiman bin Um Aiman and Aiman Ibn Um Aiman was Usama's brother from the maternal side, and he was one of the Ansar. He was seen by Ibn 'Umar not performing his bowing and prostrations in a perfect manner.So Ibn 'Umar told him to repeat his prayer.