When is it preferable to perform Witr if one fears being unable to wake up later?
Chapter on the Times When Prayer is Prohibited
Al-Mughni
Book of Prayer
Primary text
If a person fears being unable to wake up for the latter part of the night, it is recommended (Mustahabb) for them to perform Witr at the beginning of the night. The Prophet (peace be upon him) advised Abu Hurayrah, Abu Dharr, and Abu Ad-Darda' to perform Witr before sleeping, instructing them, "Whoever fears not waking up in the last part of the night, let him perform Witr at its beginning." These authentic Hadiths were narrated by Muslim and others.
Supporting text
When asked about their timing for Witr, the Prophet informed Abu Bakr to perform it early (which he described as taking the firmest approach, 'al-Hazm'), and he told Umar to perform it late (which he described as taking the strong approach, 'al-Quwwah').