Alsalam You have to wait for 20 minutes after sunrise. You can easily find the sunrise time each day on any website.
The reason why prayer at these times is not allowed is so as to avoid any resemblance to the kuffaar who prostrate to the sun when it rises as a greeting to it and to express their joy.
The Prophet (peace and blessings of Allaah be upon him) was keen to put a stop to all things that may lead to shirk, or that might involve any resemblance to the mushrikeen.(non Muslims but specifically those who don't belive in god)
The Hadith I found about this is:
“There is no prayer after the Fajr until the sun (sufficiently) rises; and there is no prayer after the ‘Asr until the sun completely sets.” [M-1041, agreed upon]
The other time mentioned here is after Asr prayer till Maghreb prayer. (Again for the same reason as before.)
Hope I helped you :)