Is a prayer devoid of khushu' accepted, and is it necessary to repeat it?

Humility (Khushu') is a state of the soul from which emanate tranquility and modesty in the limbs. It is a fear in the heart and a lowering of the gaze during prayer, or the heart's full engagement with prayer and supplication. The majority of jurists consider humility in prayer to be a Sunnah, basing their view on the Almighty's saying: "Successful indeed are the believers those who are humble in their prayers," and the Prophet's (peace be upon him) saying: "There is no Muslim who performs ablution well, then stands and prays two rak'ahs, fully attentive to them with his heart and face, except that Paradise becomes obligatory for him." Their evidence for the validity of prayer without humility is that the Prophet (peace be upon him) did not order the man who was fiddling with his beard to repeat his prayer. Some jurists, however, have gone on to say that humility is an obligation, and this is the relied-upon view among the Malikis, while others have considered it a duty, the omission of which invalidates the prayer.