Is it best to perform the witr salah before Fajr? If so, what about during the last 15 days of Ramadaan, is it best to perform this salah in jamah or alone before fajr considering that it is one's habit to perform everyday Tahajjud salah?
The time for the witr prayer enters after the performance of the nightfall (‘isha) prayer and lasts until true dawn (al-fajr al-sadiq). It is recommended to delay performing the witr prayer until the end of its time provided one is certain one will wake up before the time for dawn prayer enters. This recommendation applies to the performance of witr during Ramadhan as well, as Sheikh ‘Abd Allah al-Jurdani mentions in Fath al-‘Allam. In other words, it is optimal to postpone the witr prayer even when such delay causes one to miss performing the prayer in a group (2: 35). It should be noted that the recommendation of performing witr in a group during Ramadhan is not restricted to the last fifteen days of Ramadhan; rather, it is recommended to perform witr in a group during the entire month of Ramadhan.
And Allah knows best,
Shaykh Mahmud Adam