Reason for not looking towards heaven and looking at the place of Sujud during Salah
The basic principle is that
during Salah, Khushu` (the heart being submissively attuned to the act of worship) is obligatory and its perfection is recommended. Allah mentions it as one of the obligations of Salah in His saying:
Successful indeed are the believers.
Those who offer their Salât (prayers) with all solemnity and full submissiveness.
And those who turn away from Al-Laghw (dirty, false, evil vain talk, falsehood, and all that Allâh has forbidden).
And those who pay the Zakât.
And those who guard their chastity (i.e. private parts, from illegal sexual acts)
Furthermore, the Prophet (peace be upon him) said about the people who turn their eyes toward the heaven:
“Let some people desist from lifting their sights towards heaven when supplicating in Salah (Prayer), or else they will not return to them.”
Another narration states:
“...Or else their sights will be snatched away.”
This is because
turning the eyes toward the heaven may lead a person who is performing Salah to seeing things that distract their heart and prevent them from obtaining Khushu` and contemplating what is being recited.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.