Is the 'Tathwīb' in the Fajr prayer ("Prayer is better than sleep") recommended or obligatory, and how does its being recommended align with the jurisprudential rule: "A command necessitates obligation," given that the Prophet, peace and blessings be upon him, commanded Bilal to do it?

The 'Tathwib' (As-salatu khairun min an-nawm - Prayer is better than sleep) in the Fajr adhan (definition) has been established by authentic hadiths narrated by a number of Companions, and many scholars and jurists have affirmed its legitimacy. Those who affirm its legitimacy have differed on its ruling. The majority of them hold that it is a recommended sunnah (definition), because the command for the 'Tathwib' was not mentioned in the hadith (definition) of Abdullah ibn Zayd, which is the primary source regarding the adhan. Others have held it to be obligatory, acting upon the apparent command, because the Prophet, peace and blessings be upon him, taught it to Abu Mahdhura afterwards. The prevailing view is that it is legitimate and a sunnah; if it is omitted, the adhan is still valid, though the virtue is lost.