Caution: Translations of Quran and Ḥadīth may lead to possible misapplications and misinterpretations. This site is intended for students of sacred knowledge that are proficient in comprehending classical Arabic and have a strong foundation in Islamic sciences. Also note that religious injunctions rely on several aspects beyond what one may glean through reading individual aḥādīth.
Qurʾān Bukhārī Muslim Nasāʾī Abū Dāwūd Tirmidhī Ibn Mājah Mālik Aḥmad Dārimī Ḥākim Ibn Ḥibbān Ṭabarānī Nasāʾī's Kubrá Bayhaqī al-Luʾluʾ wa-al-Marjān Ibn Rajab's Fifty Riyāḍ al-Ṣāliḥīn Aḥmad's Zuhd Suyūṭī
malik:2-8 – Mālik > Zayd b. Aslam > ʿUmar b. al-Khaṭṭāb

Yahya related to me from Malik from Zayd ibn Aslam that Umar ibn al-Khattab said, "If you fall asleep Iying down you must do wudu ( before you pray) . "  

مالك:٢-٨ – وَحَدَّثَنِي عَنْ مَالِكٍ عَنْ زَيْدِ بْنِ أَسْلَمَ أَنَّ عُمَرَ بْنَ الْخَطَّابِ

قَالَ إِذَا نَامَ أَحَدُكُمْ مُضْطَجِعًا فَلْيَتَوَضَّأْ  

See similar narrations below:

Collected by Bayhaqī
bayhaqi:581 – Abū ʿAbd al-Raḥman al-Sulamī And ʾAbū Naṣr ʿUmar b. ʿAbd al-ʿAzīz b. Qatādah > Abū ʿAmr b. Nujayd > Muḥammad b. Ibrāhīm > Ibn Bukayr > Mālik > Zayd b. Aslam > ʿUmar b. al-Khaṭṭāb

[Machine] If one of you sleeps while lying down, then let him perform ablution. This is a message.  

البيهقي:٥٨١ – أَخْبَرَنَا أَبُو عَبْدِ الرَّحْمَنِ السُّلَمِيُّ وَأَبُو نَصْرٍ عُمَرُ بْنُ عَبْدِ الْعَزِيزِ بْنِ قَتَادَةَ قَالَا نا أَبُو عَمْرِو بْنُ نُجَيْدٍ نا مُحَمَّدُ بْنُ إِبْرَاهِيمَ نا ابْنُ بُكَيْرٍ عَنْ مَالِكٍ عَنْ زَيْدِ بْنِ أَسْلَمَ أَنَّ عُمَرَ بْنَ الْخَطَّابِ قَالَ

إِذَا نَامَ أَحَدُكُمْ مُضْطَجِعًا فَلْيَتَوَضَّأْ هَذَا مُرْسَلٌ