Muwatta Malik 38
Mauquf Daif Graded by Salim al-Hilali Book of Purity · Hadith 38 of 1,858وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ زَيْدِ بْنِ أَسْلَمَ، أَنَّ عُمَرَ بْنَ الْخَطَّابِ، قَالَ إِذَا نَامَ أَحَدُكُمْ مُضْطَجِعًا فَلْيَتَوَضَّأْ .
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)
References
Muwatta Malik 38
In-book: Book 2, Hadith 7 Source: hadith-api (fawazahmed0)