Dear Sheikh, Salaam Alaikum .. Kindly advise concerning the authenticity of the below hadith putforward on the DARUL IFTA site http://darulifta-deoband.org/viewfatwa.jsp?ID=7636.
"There are three characteristics of Prophethood: hastening Iftaar (the breaking of the fast), deferring Sahoor (the pre-dawn meal)) and putting the hands under the navel during the prayer." Jazaak Allaahu Khairan..

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

Abu Ad-Dardaa’ narrated that the Prophet said: ''There are three characteristics of Prophethood: hastening Iftaar (the breaking of the fast), deferring sahoor (the pre-dawn meal)] and putting the right hand over the left hand under the navel." [At-Tabaraani, and Al-Albaani classified it as Saheeh [sound], however, this is without the addition “…and putting the right hand over the left hand under the navel.”

Al-Haythami said in “Majma’ Az-Zawaai’d”: “This Hadeeth was reported by At-Tabaraani in Al-Kabeer as quoted from the Companion attributing it to the Prophet and as said by the Companion Abu Ad-Dardaa’ without attributing it to the Prophet, sallallaahu ‘alayhi wa sallam. The Hadeeth said by the Companion is authentic and the one that was narrated by the Companion attributing it to the Prophet, sallallaahu ‘alayhi wa sallam, have in its chain of narrators some men whom I did not find any biography about them.”

As regards the narration by Anas which you mentioned in the question, it was reported by Al-Jawhari in his book “Amaali Al-Jawhari” as clarified by Al-Muttaqi Al-Hindi, the author of “Kanz Al-‘Ummaal”; however, without the addition that is mentioned.

Besides, Al-Muttaqi Al-Hindi mentioned this narration that ‘Ali said without attributing it to the Prophet, sallallaahu ‘alayhi wa sallam: “There are three characteristics of Prophethood: hastening Iftaar (the breaking of the fast), deferring Sahoor (the pre-dawn meal)) and putting the hands under the navel during the prayer.” Al-Muttaqi Al-Hindi attributed it to Ibn Shaaheen and Abu Muhammad Al-Ibraaheem in the book of the prayer.

Besides, An-Nawawi said in his commentary on "Saheeh Muslim", "The Hadeeth narrated from Ali that it is a Sunnah in the prayer to put one hand over the other in the prayer under the navel is Dha’eef [weak] and it is agreed (among the scholars) that it is Dha’eef."

Moreover, Al-Mubaarak Poori said: “Shaykh Haashim As-Sindi said: "As regards the narration about putting the right hand over the left hand under the navel in the prayer, I did not come across the chain of narrators of this Hadeeth, but Az-Zaahidi added that ‘Ali Ibn Abi Taalib narrated it from the Prophet but Ibn Ameer Al-Haajj and Ibn Nujaym, said: The scholars of Hadeeth did not come across the expression “under the navel” either as reported from the Prophet or from the Companion without attributing it to the Prophet, sallallaahu ‘alayhi wa sallam.” [End of quote of Haashim As-Sindi]

Hence, it is the above Ahaadeeth that were given as evidence for putting the hands under the navel in the prayer and you have known that they are not suitable to be presented as evidence for that.

Allaah Knows best.