Which of the following prayers is not observed between Isha and Fajr?

The prayer that is not observed between Isha and Fajr is Istisqa. Istisqa is a special prayer for asking for rain and is performed only when there is a need for rain in a specific area. It can be performed at any time during the day or night, and is not limited to the time between Isha and Fajr. Witr is a special, optional prayer that is performed after Isha and before Fajr. Tahajjud is a late-night prayer that can be performed after Isha and before Fajr. Shaf'i is a term used to describe one of the four main schools of thought in Sunni Islam, and it does not refer to a specific prayer.