one could - however- consider them as evidences [on the obligation of Tashahhud] for any order indicates that the thing ordered is an obligation. If parts of any order are not an obligation due to a certain evidence this does not entail that the other parts are not. All?h knows best.
Our reverend Sh?fi‘? scholars considered saying: as ‘All?humma Sall? ‘Al? Muhammad’ W?jib, while the rest of Tashahhud is Sunnah. There is an odd opinion that states the obligation of sending Sal?t on the ‘?l [house hold] but it is not strong enough to be taken into consideration. All?h ( - عز وجل -) knows best’. (1)
[Commenting on An-Nawawi’s last point,] Al-’Am?r As-San’?n? said: ‘Sending one’s Sal?t on the ‘?l is not just recommended as An-Nawaw? and others stated. On the contrary; sending Sal?t on the prophet ( - صلى الله عليه وسلم -) is not complete unless a muslim recites the whole wording of it which includes the ‘?l for [it is narrated that] the Sah?b? asked the prophet ( - صلى الله عليه وسلم -) regarding how to send Sal?t on him ( - صلى الله عليه وسلم -) , and he ( - صلى الله عليه وسلم -) taught him the whole wording that contains sending Sal?t on both: the prophet ( - صلى الله عليه وسلم -) and his house hold.
Accordingly; whoever ignores the second part, has not fully complied with the prophet’s ( - صلى الله عليه وسلم -) order, and so he has not actually sent Sal?t on him ( - صلى الله عليه وسلم -) ’. (2)
(1) “ Sharh Sah?h Muslim” by An-Nawaw? (vol. 4 / p. 123), see also: “Fathul B?r?” (vol. 11 / p. 163 .. ).
(2) See: “Subulus Sal?m” (vol. 1 / p. 193). Al-H?d?, Al-Q?sim, Ahmad Ibn Hanbal and some Sh?fi‘? scholars all held it an obligation to recite the full form of the Sal?t on the prophet ( - صلى الله عليه وسلم -) after reciting Tashahhud as stated in “Naylul Awt?r” (vol. 2 / p. 324).
In “Al-Qawlul Bad?‘“ (pp. 90 - 1), As- Sakh?w? related from Al-Baihaqi in his “Shu‘ab” from Ab? Ish?q Al-Marwazi - a great Sahfi‘? scholar - to have said: ‘ I believe that sending Sal?t on the prophet ( - صلى الله عليه وسلم -) and his ’?l is an obligation in the last Tashahhud of Sal?t. Al-Baihaq? commented: “The affirmed ah?d?th that show the way of sending Sal?t on the prophet ( - صلى الله عليه وسلم -) are evidences on what he believed in”.
Then he [As-Sakh?w?] said: ‘Our shaikh -Ibn Hajar- said that what At-Tah?w? mentioned in his “Mushkil” indicates that Harmalah reported this from Ash- Sh?fi‘?’.