It is narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) said: "When any one of you wakes up from sleep, he should wash his hands before putting them into water, for he does not know where his hand spent the night."

It is narrated from Hazrat Zaid bin Aslam that he said: Sayyiduna Umar bin Khattab (may Allah be pleased with him) said: Whoever among you sleeps lying down should perform ablution.

Hazrat Zaid bin Aslam said that what Allah, Glorious and Exalted, has said: "When you rise for prayer, then wash your face, and your hands up to the elbows, and wipe your heads, and wash your feet up to the ankles." The purpose of this is that when you rise for prayer after sleeping.

Imam Malik (may Allah have mercy on him) said: In our view, nosebleed, blood flowing, or pus oozing does not necessitate ablution; rather, one should not perform ablution except due to impurity that comes out from the anus or the penis, or if one sleeps.

It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that he used to fall asleep while sitting, then he would perform prayer and would not perform ablution.