"He (صلى الله علیه وسلم) used to incline his head during prayer and fix his sight towards the ground"  - "while he was in the Ka'bah, his sight did not leave the place of his prostration until he came out from it" ; and he said,
It is not fitting that there should be anything in the House which disturbs the person praying. 
"He used to forbid looking up at the sky" , and he emphasised this prohibition so much that he said:
People must refrain from looking up at the sky in prayer, or their sight will not return to them (and in one narration: or their sight will be plucked away). 
In another hadeeth:
"So when you pray, do not look here and there, for Allaah sets His Face for the face of his slave in his prayer as long as he does not look away." 
And he also said about looking here and there,
"It is a snatching away which the devil steals from the slave during prayer." 
He (صلى الله علیه وسلم) also said:
"Allaah does not cease to turn to a slave in his prayer as long as he is not looking around; when he turns his face away, Allaah turns away from him. 
He "forbade three things: pecking like a hen, squatting (iq'aa'), like a dog and looking around like a fox" ; he also used to say,
"Pray a farewell prayer as if you see Him, but if you do not see Him, surely He sees you" 
"Any person who, when an obligatory prayer is due, excels in its ablution, humility and bowings, will have it as a remission for his previous minor sins as long as he does not commit a major sin, and this (opportunity) is for all times." 
Once he (صلى الله علیه وسلم), prayed in a khameesah  and (during the prayer) he looked at its marks. So when he finished, he said:
Also "'Aaishah had a cloth with pictures spread towards a sahwah , towards which the Prophet (صلى الله علیه وسلم) prayed and then said:
"Take it away from me [for its pictures did not cease to thwart me in my prayer]." 
He would also say:
"Prayer is not valid when the food has been served, nor when it is time to relieve oneself of the two filths." 
 Baihaqi and Haakim, who declared it saheeh and it is as he said. It also has a strengthening hadeeth reported by ten of his Companions: transmitted by Ibn `Asaakir (17/202/2). See Irwaa' (354).
NOTE: These two ahaadeeth show that the sunnah is to fix one's sight on the place of prostration on the ground, so the action of some worshippers of closing their eyes during Prayer is misdirected piety, for the best guidance is the guidance of Muhammad (صلى الله علیه وسلم).
 Abu Daawood & Ahmad with a saheeh isnaad (Irwaa', 1771); what is meant here by 'the House' is the Ka'bah, as the context of this hadeeth shows.
 Bukhaari & Abu Daawood.
 Bukhaari, Muslim & Siraaj.
 Tirmidhi & Haakim, who declared it saheeh, cf. Saheeh at-Targheeb (no. 353)
 Bukhaari and Abu Daawood.
 Transmitted by Abu Daawood and others. Ibn Khuzaimah and Ibn Hibbaan declared it saheeh. See Saheeh at-Targheeb (no.555)
 Ahmad and Abu Ya`laa. See Saheeh at-Targheeb (no. 556).
 Mukhlis in Ahaadeeth Muntaqaah, Tabaraani, Rooyaani, Diyaa' in al-Mukhtaarah, Ibn Maajah, Ahmad and Ibn `Asaakir. Haitami declared it saheeh in Asnaa al-Mataalib.
 A woollen garment having marks.
 A coarse garment without marks.
 Bukhaari, Muslim & Maalik. It is given in Irwaa' (376).
 "A small room embedded in the ground slightly, like a small chamber or cupboard" (Nihaayah).
 Bukhaari, Muslim & Abu `Awaanah. The Messenger (صلى الله علیه وسلم) did not order the wiping out or tearing of the pictures but only removed them because - and Allaah knows best - they were not pictures of things having souls. The evidence for this is that he (صلى الله علیه وسلم) tore other pictures as proved by many narrations in Bukhaari and Muslim, and whoever wishes to explore this further should consult Fath al-Baari (10/321) and Ghaayah al-Maraam fi Takhreej Ahaadeeth al-Halaal wal-Haraam (nos. 131-145).
 Bukhaari and Muslim.