Yahya related to me from Malik from Zayd ibn Aslam that the ayat "You who believe! When you rise for prayer wash your faces, and your arms to the elbows, and wipe over your heads and your feet up to the ankles," refers to rising from bed, meaning sleep. Yahya said that Malik said, "The situation with us is that one does not have to do wudu for a nose-bleed, or for blood, or for pus issuing from the body. One only has to do wudu for impurities which issue from the genitals or the anus, or for sleep."  

أَنَّ تَفْسِيرَ هَذِهِ الآيَةِ يَا أَيُّهَا الَّذِينَ آمَنُوا إِذَا قُمْتُمْ إِلَى الصَّلاَةِ فَاغْسِلُوا وُجُوهَكُمْ وَأَيْدِيَكُمْ إِلَى الْمَرَافِقِ وَامْسَحُوا بِرُءُوسِكُمْ وَأَرْجُلَكُمْ إِلَى الْكَعْبَيْنِ أَنَّ ذَلِكَ إِذَا قُمْتُمْ مِنَ الْمَضَاجِعِ يَعْنِي النَّوْمَ قَالَ يَحْيَى قَالَ مَالِكٌ الأَمْرُ عِنْدَنَا أَنَّهُ لاَ يَتَوَضَّأُ مِنْ رُعَافٍ وَلاَ مِنْ دَمٍ وَلاَ مِنْ قَيْحٍ يَسِيلُ مِنَ الْجَسَدِ وَلاَ يَتَوَضَّأُ إِلاَّ مِنْ حَدَثٍ يَخْرُجُ مِنْ ذَكَرٍ أَوْ دُبُرٍ أَوْ نَوْمٍ