has indeed incurred a sin upon him self though his Sal?t is correct. (1)
According to the Fiqh principle which states that ‘Forbidding an act entails its invalidity’, (2) Ibn ‘Umar and Im?m Ahmad considered that the Sal?t done in the previously mentioned manner as invalid.
Different interpretations were provided regarding the transformation [of the head] mentioned in Ab? Hurairah’s narration some of which are the following:
(a) Some say that the words mean their literal apparent meaning; the head is transformed into that of donkey. Using the future tense in the had?th supports this view.
Some may argue that nothing in the had?th indicates that the transformation of a head is really going to happen but the purport of the had?th is that one is likely to fall under such a threat. This is indeed a false view for nothing prevents the threat to really take place.
(b) Some say that this threat is going to take place on the Day of Qiy?mah [Resurrection].
(c) Some held that the defacement refers to the abstract meaning of the word; that is extinguishing all goodness in one’s heart to an extent that one is unable to see the path of the Truth and guidance and hence going astray. (3)
(1) Some Fuqah?’ stated that a Ma’m?m’s Sal?t is considered invalid if he / she preceded his / her Im?m with two practical acts of Sal?t or delayed doing them. ... =
= ... No doubet that in doing so, one incurs a sin on himself for contradicting the Sunnah-as was previously mentioned-but there is no hard evidence that one’s Sal?t is invalid; the Sah?bah for example followed the prophet ( - صلى الله عليه وسلم -) when he performed the fifth Rak‘ah which includes many Athk?r and did not order them to repeat their Sal?t. Accordingly, many of the rulings [some of] the Fuqah?’ deliver regarding the invalidity of some acts of worship are improper, sometimes. What is the difference between preceding the Im?m in one or two acts or between verbal and physical ones?? See: “As-Saylul Jarr?r” (vol. 1 / pp. 272 - 3).
(2) See: The treatise titled with “As-Sal?t” by Im?m Ahmad (pp. 37 - 8).
(3) See: “Sharh Sah?h Muslim” by An-Nawaw? (vol. 4 / pp. 150 - 1), “Fathul B?r?” (vol. 2 / p. 184) and “Ad-Dinul Kh?lis” (vol. 3 / p. 73 - 4).