he saw the Prophet sleeping, while in the prostration position, until he snored or snorted. Then he stoodup to pray. So I said: "O Messenger of Allah! You were sleeping?" He said: "Wudu is not required except for sleeping while reclining. For when one reclines, joints relax."  

أَنَّهُ رَأَى النَّبِيَّ ﷺ نَامَ وَهُوَ سَاجِدٌ حَتَّى غَطَّ أَوْ نَفَخَ ثُمَّ قَامَ يُصَلِّي فَقُلْتُ يَا رَسُولَ اللَّهِ إِنَّكَ قَدْ نِمْتَ قَالَ إِنَّ الْوُضُوءَ لاَ يَجِبُ إِلاَّ عَلَى مَنْ نَامَ مُضْطَجِعًا فَإِنَّهُ إِذَا اضْطَجَعَ اسْتَرْخَتْ مَفَاصِلُهُ  

"The companions of Messenger of Allah would sleep, then stand to pray, They would not perform Wudu."  

كَانَ أَصْحَابُ رَسُولِ اللَّهِ ﷺ يَنَامُونَ ثُمَّ يَقُومُونَ فَيُصَلُّونَ وَلاَ يَتَوَضَّئُونَ