عَنْ عَلِيٍّ رَضِيَ اللَّهُ عَنْهُ قَالَ: كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَآلِهِ وَسَلَّمَ إِذَا رَكَعَ لَوْ وَضِعَ قَدْحٌ مِنْ مَاءٍ عَلَى ظَهْرِهِ لَمْ يُهْرَقْ
Sayyiduna Ali (may Allah be pleased with him) says that when the Messenger of Allah (peace and blessings be upon him) performed ruku‘, he would be so straight that if a bowl of water were placed on his back, it would not spill.