When one of you dozes in prayer he should sleep till his sleep is gone, for when one of you prays while he is dozing, perhaps he might curse himself if he begs pardon of Allah.  

أَنَّ النَّبِيَّ ﷺ قَالَ إِذَا نَعَسَ أَحَدُكُمْ فِي الصَّلاَةِ فَلْيَرْقُدْ حَتَّى يَذْهَبَ عَنْهُ النَّوْمُ فَإِنَّ أَحَدَكُمْ إِذَا صَلَّى وَهُوَ نَاعِسٌ لَعَلَّهُ يَذْهَبُ يَسْتَغْفِرُ فَيَسُبَّ نَفْسَهُ  

The Messenger of Allah ﷺ as saying: When one of you gets up by night (to pray), and falters in reciting the Qur'an (due to sleep), and he does not understand what he utters, he should sleep.  

قَالَ رَسُولُ اللَّهِ ﷺ إِذَا قَامَ أَحَدُكُمْ مِنَ اللَّيْلِ فَاسْتَعْجَمَ الْقُرْآنُ عَلَى لِسَانِهِ فَلَمْ يَدْرِ مَا يَقُولُ فَلْيَضْطَجِعْ  

The Messenger of Allah ﷺ entered the mosque (and saw that) a rope tied between two pillars. He asked: What is this rope (for) ? The people told him: This is (for) Hamnah b. Jahsh who prays (here). When she is tired, she reclines on it. The Messenger of Allah ﷺ said: She should pray as much as she has strength. When she is tired, she should sit down. This version of Ziyad has: He said: What is this ? The people told him: This is for Zainab who prays. When she becomes lazy, or is tired, she holds it. He said: Undo it. One of you should pray in good spirits. When he is lazy or tired, he should sit down.  

دَخَلَ رَسُولُ اللَّهِ ﷺ الْمَسْجِدَ وَحَبْلٌ مَمْدُودٌ بَيْنَ سَارِيَتَيْنِ فَقَالَ مَا هَذَا الْحَبْلُ فَقِيلَ يَا رَسُولَ اللَّهِ هَذِهِ حَمْنَةُ بِنْتُ جَحْشٍ تُصَلِّي فَإِذَا أَعْيَتْ تَعَلَّقَتْ بِهِ فَقَالَ رَسُولُ اللَّهِ ﷺ لِتُصَلِّ مَا أَطَاقَتْ فَإِذَا أَعْيَتْ فَلْتَجْلِسْ قَالَ زِيَادٌ فَقَالَ مَا هَذَا فَقَالُوا لِزَيْنَبَ تُصَلِّي فَإِذَا كَسِلَتْ أَوْ فَتَرَتْ أَمْسَكَتْ بِهِ فَقَالَ حُلُّوهُ فَقَالَ لِيُصَلِّ أَحَدُكُمْ نَشَاطَهُ فَإِذَا كَسِلَ أَوْ فَتَرَ فَلْيَقْعُدْ