Why did the jurists not adopt Ibn Mas'ud's action concerning his two Rak'ahs of Fajr prayer when the prayer was established, whereas some scholars used Ibn 'Umar's action as evidence regarding taking what exceeded a handful from the beard?

The jurists differed on the authoritative force of a Companion's saying. If what the Companion said is something in which personal opinion has no scope, then it is considered as if it were a Marfu‘ (a Hadith (definition) directly attributed to the Prophet). If, however, there is scope for personal opinion, and it became widespread among the Companions with no one opposing it, then it is a silent consensus (ijma‘ sukuti) and is considered a proof by the majority. But if a Companion's saying contradicts an authentic Sunnah (definition) and the actions of other Companions, then it is not acted upon.

Accordingly, what was reported from Ibn Mas‘ud (may Allah be pleased with him) concerning his praying the two rak‘ahs of Fajr while the Imam was praying contradicts the authentic Sunnah established by the Prophet’s (peace be upon him) saying: «When the prayer is called, there is no prayer except the obligatory one,» and by his (peace be upon him) action, and by the action of Ibn ‘Umar and other Companions. If the Sunnah of Fajr is missed, it can be prayed after sunrise or before it. As for what is authentically reported from a Companion without contradiction from others, it is considered a proof.