What to say when lying down to sleep

It is narrated from al-Sa'ib (may Allah have mercy on him) that he said: I was with Sayyiduna Ammar bin Yasir (may Allah be pleased with him) when a man came to him and said: "Shall I not teach you some words?" It was as if he was attributing those words to the Prophet (peace and blessings be upon him). Then he said: "When you lie down on your bed at night, say: 'Allahumma inni aslamtu nafsi ilayk, wa wajjahtu wajhi ilayk, wa fawwadtu amri ilayk, wa alja'tu zahri ilayk, amantu bi-kitabika al-munazzal, wa bi-nabiyyika al-mursal. Allahumma nafsi khalaqtaha, laka mahyaha wa laka mamatuha, in qabadtaha farhamha, wa in akhkhartaha fahfazha bi-hifz al-iman' (O Allah, I have surrendered my soul to You, turned my face towards You, entrusted my affair to You, and committed my back to Your refuge (in reliance); I believe in Your Book which was sent down and in Your Prophet who was sent. O Allah, You created my soul; its life and its death are in Your control. If You take it (during sleep), have mercy on it, and if You grant it further respite of life, protect it with faith)."
The chain of this hadith is hasan (good), and there are supporting reports (shawahid) for it in Sahih al-Bukhari and Muslim from the narration of Sayyiduna al-Bara' bin Azib (may Allah be pleased with him) and from other Companions.

It is narrated from Sayyiduna Anas (may Allah be pleased with him) that a woman came to the Prophet (peace and blessings be upon him) and complained to him of her financial hardship and need. The Messenger of Allah (peace and blessings be upon him) said: "Shall I not guide you to something that is better for you than this (worldly need)? When you go to sleep, declare the Oneness of Allah Almighty thirty-three times, that is, say 'La ilaha illallah' (There is no god worthy of worship except Allah); glorify Him thirty-three times, that is, say 'Subhanallah' (Allah is free from every imperfection); and praise and extol Him thirty-four times, that is, say 'Alhamdulillah' (All praise belongs to Allah). These complete hundred words are far better for you than the world and all the blessings in it."
Imam Abu Bakr bin Abi Shaybah narrated this hadith. Although the narrator Salamah bin Wardan in its chain is da'if (weak), the virtues of these glorifications at bedtime are also established by other authentic narrations in the two Sahihs.

It is narrated from Sayyiduna Mu'adh bin Jabal (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever sleeps in a state of ablution and then wakes up at some hour of the night, whatever he asks Allah Almighty of the affairs of this world and the Hereafter, Allah Almighty will certainly give it to him."
Imam Thabit (may Allah have mercy on him) relates: Then the elder from whom Shahr bin Hawshab had narrated this hadith to us came to us, and he himself narrated this same hadith to us.

It is narrated from Sayyidah Aishah (may Allah be pleased with her): "The Messenger of Allah (peace and blessings be upon him) would give instructions about his bedding, and it would be spread out for him; then he (peace and blessings be upon him) would face the qiblah. When he (peace and blessings be upon him) came to his bed to rest, he would make his right palm a pillow, then he would recite something in a low voice, and we did not know what he (peace and blessings be upon him) was saying. Then, when he reached the last part of it, he would raise his voice and supplicate with this du'a: 'Allahumma rabba al-samawati al-sab', wa rabba al-'arshi al-'azim, ilaha wa rabba kulli shay', munzila al-Tawrati wa al-Injili wa al-Furqan, faliqa al-habbi wa al-nawa, a'udhu bika min sharri kulli shay'in anta akhidhun bi-nasiyatih. Allahumma anta al-awwalu fa-laysa qablaka shay', wa anta al-akhiru fa-laysa ba'daka shay', wa anta al-zahiru fa-laysa fawqaka shay', wa anta al-batinu fa-laysa dunaka shay', iqdi 'anna al-dayna wa aghnina min al-faqr' (O Allah, Lord of the seven heavens and Lord of the Mighty Throne, God and Lord of everything, Revealer of the Torah, the Gospel and the Furqan (the Qur'an), Splitter of the grain and the date-stone (so that they sprout), I seek refuge in You from the evil of everything whose forelock is in Your grasp. O Allah, You are the First, so there is nothing before You; You are the Last, so there is nothing after You; You are the Manifest, so there is nothing above You; and You are the Hidden, so there is nothing nearer than You. Settle our debt for us, and free us from poverty and need and make us self-sufficient)."
Imam Abu Ya'la narrated this hadith in his Musnad; al-Sari bin Isma'il in its chain is a weak narrator, but these words of the du'a are narrated in Sahih Muslim from Sayyiduna Abu Hurayrah (may Allah be pleased with him).