What is the ruling on someone who woke up late for Asr prayer, such that he feared having entered the time of necessity, and is his prayer valid?

The "necessary time" for Asr begins with the yellowing (of the sun), which is when the sun's color appears yellow on the ground and walls. It is not permissible to delay the prayer until this time without an excuse. A person who sleeps after the entry of the time for prayer does not sin if he expects to wake up before its exit. However, he sins if he does not expect to wake up until after its exit.