According to the Hanafi Madhab and some of the Shaafiees, it is Mustahabb (preferable) for one to make Masah (pass wet hands) of his nape in Wudhu. This is based upon adequate proofs, some being authentic and others weak. However, after all of these proofs are combined they become strong enough to prove this issue.
1.Imaam Abu Dawud (RA) and Imaam Ahmad ibn Hambal (RA) have recorded a narration in their respective works that Rasulullah (Sallallaahu Alayhi Wasallam), whilst making Wudhu, had also madeMasah of his nape. (Sunan Abu Dawud; Musnad Ahmad )
This Hadith has been classified as weak by some Muhadditheen (refer Talkheesul Habeer of Hafiz ibn Hajar vol.1 pg.287-288; Allaamah Shakwaani has said that Hafiz ibn Sayyid-un-Naas had classified it as?Hasan(sound) see Naylul Awtaar vol.1 pg.179). Hafiz ibn Hajar (RA) has stated that,based on this narration, Imaam Muhaddith al-Baghawi al-Shaafiee has mentioned that Masah of the nape is Mustahabb. This indicates that this narration is sufficient enough to prove the issue at hand although it is weak.
2.Sayyiduna ibn Umar (Radhiallaahu Anhu) reports that Rasulullah (Sallallaahu Alayhi Wasallam) said,Whoever makesMasah of the napein Wudhu will be saved from the burning thirst / shackles on the day of Qiyaamat.(Musnadul Firdaws vol.4 pg.44 Hadith5628; Taarikh Isbahan vol.2 pg.115
This narration has been classified as weak by Hafiz al-Iraaqi and Allaamah Zabeeni (refer Takhreejul Ihyaa vol.1 pg.82 Hadith308; Sharhul Ihyaa vol.2 pg.580; Ilmiyyah)
3.Imaam ibn Faaris (RA) has also narrated this narration of Sayyiduna Abdullah ibn Umar (Radhiallaahu Anhu) with a separate chain of narrators and Imaam ibn Faaris then said,This, Insha Allah, is an authentic (Sahih) Hadith (refer Talkhisul Habeer vol.1 pg.288)
4.Imaam Abu Ubayd (RA) has recorded a narration from Imaam Musa ibn Talha that,Whoever makes?Masah of his napetogether with his head will be saved from severe thirst / shackles on the day of Qiyaamat (refer Talkhisul habeer vol.1 pg.288; Sharh al-Ihyaa vol.2 pg.580)
Muhaddith Zafar Ahmad al-Uthmaani (RA) has accepted all the narrators of this narration to be reliable. (Ielaa-us-sunan vol.1 pg.123)
Those who deny the existence of such Ahaadith have relied upon the claim of some scholars of the past, i.e. Imaam al-Nawawiy and Imaam ibn Qayyim (RA), that there exists no Hadith of this kind.
However, as you have seen, there exists numerous Ahaadith that give this meaning. It is for this reason that some Muhadditheen and Fuqahaa that came later on had rejected the claim of Imaam al-Nawawiy, etc. We would be forced to say that perhaps Imaam al-Nawawiy (RA) did not come across these narrations and this is what lead him to that decision. In fact, Allaamah Shawkaani (RA), after mentioning the abovementioned narrations and a few others, states:And by means of all these narrations you would acknowledge that the claim of Imaam al-Nawawiy was irrational and a result of recklessness (Shawkaani further states) even Imaam ibn Rifah (RA) disagreed with Imaam al-Nawawiy (RA) and he argued that Imaam Baghawiy (RA) who is an accepted Muhaddith had said that it was Mustahabb to make Masah on the nape and he (Imaam Baghawiy) would only base his decision on a narration nothing else. (Naylul Awtaar vol.1 pg.179; Ilmiyyah)
As we have stated at the outset, that though some narrations are weak, there are others that are authentic as well, like the ones referred to in numbers 3 and 4, and some Muhadditheen have also regarded number 1 as authentic.
Besides the above, there are other narrations mentioned by Allaamah Shawkaani (RA) in Naylul Awtaar.
Moreover, there exists another narration that has somehow eluded those who reject the validity of Masah on the nape, that is the following:
5.Sayyiduna Waail ibn Hujr (Radhiallaahu Anhu) has reported a lengthy narration regarding the manner in which Rasulullah (Sallallaahu Alayhi Wasallam) performed Wudhu, in which he stated, And he (Sallallaahu Alayhi Wasallam) hadmade Masah on his napeand the inner part of his beard with the water that remained from the Masah of the head. (This narration has been recorded by Imaam al-Tabrani and Bazzaar).
Hafiz al-Iraaqi (RA), the Ustaadh (teacher) of the famous Hafiz ibn Hajar (RA), referred to this Hadith with the following words,Yes, the mention of Masah on the nape has been reported in the narration of Sayyiduna Waail ibn Hujr (Radhiallaahu Anhu) by Imaam al-Tabrani in his Mujamul-Kabeer and Imaam Bazzaar in his Musnad and there is nothing wrong with its chain of narrators. (Tanzeehus-shareeah vol.2 pg.75). Hence, this Hadith according to Hafiz al-Iraaqi is effectively classified as Hasan (soundly authenticLaa ba-asa bihi.
Therefore, there remains no doubt concerning the validity of the Hanafi standpoint on this matter. It is surprising to note that those who claim to follow Hadith, openly deny it and falsely claim that the Hanafi Madhab is not based on Hadith, but on rational only.
6. In fact, some scholars have even proven the Masah of the nape through those Ahaadith in which Rasulullah (Sallallaahu Alayhi Wasallam) has stated that his followers will be recognized on the day of Qiyaamat through the shine on their limbs of Wudhu. Rasulullah (Sallallaahu Alayhi Wasallam) then said: If any one is able to increase that shine, he should do so., i.e. by washing more than the required amount. (Sahih al-Bukhari Hadith136 and Sahih Muslim Hadith579) For example, when washing the arms, to wash till way above the elbows and when washing the feet, to do so till the shin, etc. So, in the same manner, if one, while making Masah, increases it till his nape, he will be following this Hadith as well. (refer to:Tuhfatul Kamalah alaa hawaashi Tuhfatul-Talabah pg.13; Majmoo rasaail lakhnawi vol.1 pg.261