When one of you feels drowsy during prayer, let him lie down till drowsiness goes away from him
Authentic hadithعن عائشة رضي الله عنها مرفوعاً: «إذا نَعَسَ أحدكم وهو يصلي فَلْيَرْقُدْ حتى يذهب عنه النوم، فإن أحدكم إذا صلى وهو نَاعِسٌ لا يدري لعله يذهب يستغفر فَيَسُبُّ نَفْسَهُ».
‘Ā’ishah (may Allah be pleased with her) reported that the Prophet (may Allah's peace and blessings be upon him) said: "When one of you feels drowsy during prayer, let him lie down till drowsiness goes away from him, because when one of you performs prayers while feeling sleepy, unknowingly instead of seeking forgiveness he might abuses himself."
Explanation
It is disliked to overburden oneself with worship. If a worshiper feels that sleepiness will overcome him while praying, he should discontinue his prayer or complete it, then lie down and rest lest he unintentionally supplicates against himself because of exhaustion.
Source · Narrated by Bukhari & Muslim
From the Hadith Encyclopedia (hadeethenc.com).