Please tell some clear evidence for reciting Durud Sharif in the first tashahhud.
Wa alaikum assalam wa rahmatullah wa barakatuh — reciting Durud Sharif after the at-Tahiyyat in the first tashahhud (al-qu'ud al-awwal) is prescribed and established by clear and strong evidences (the position of Imam ash-Shafi'i, Shaykh al-Albani and other verifying scholars).
Wa alaikum assalam wa rahmatullah wa barakatuh — reciting Durud Sharif after the at-Tahiyyat in the first tashahhud (al-qu'ud al-awwal) is prescribed and established by clear and strong evidences (the position of Imam ash-Shafi'i, Shaykh al-Albani and other verifying scholars).
1. The explicit hadith of Sahih Muslim (Sayyidah 'A'ishah, may Allah be pleased with her): the Prophet ﷺ would pray eight rak'ahs of tahajjud/witr; when he sat at the eighth, he would remember Allah, praise Him and supplicate, «وَيُصَلِّي عَلَى نَبِيِّهِ» (and he would send blessings upon His Prophet), then he would stand for the ninth without giving salam (Sahih Muslim: 746). The point of inference: this sitting was not the one with salam but the middle/first tashahhud, and blessings were duly recited in it.
2. The generality of the hadiths of Durud: upon the revelation of the verse of salah (al-Ahzab), the Companions asked: the salam (at-Tahiyyat) we have come to know, «فَكَيْفَ نُصَلِّي عَلَيْكَ فِي صَلَاتِنَا؟» (so how should we send blessings upon you in our prayer?) — so he ﷺ taught the blessings without any distinction: «قُولُوا: اللَّهُمَّ صَلِّ عَلَى مُحَمَّدٍ...» (Say: O Allah, send blessings upon Muhammad...) (Sahih Ibn Khuzaymah: 711, ad-Daraqutni, al-Bayhaqi). No restriction was imposed that it be recited only in the last tashahhud; a general command remains upon its generality until a specific evidence of prohibition comes.
3. The teaching of Ibn Mas'ud, may Allah be pleased with him: an-Nasa'i and at-Tabarani transmitted with an authentic chain that the Prophet ﷺ taught the words of the tashahhud in the middle of the prayer (the first tashahhud) and at the end, together with encouragement towards blessings and supplication — the form of the tashahhud is the same in both places.
4. The position of the imams: Imam ash-Shafi'i (al-Umm): reciting «اللَّهُمَّ صَلِّ عَلَى مُحَمَّدٍ» (O Allah, send blessings upon Muhammad) in the first tashahhud is prescribed and recommended. Al-Albani (Sifat Salat an-Nabi ﷺ): from the authentic hadiths, blessings in the first tashahhud are Sunnah and established; there is no prohibition. The majority (Hanafis and Hanbalis): on the basis of the narration of brevity in the first sitting of the obligatory prayer, they say restricting oneself to the tashahhud alone is better, but in nafl and non-emphasized sunnah prayers they declare blessings in the first sitting permissible and good.
Summary: blessings in the first tashahhud (at least "Allahumma salli 'ala Muhammad" or the complete Durud Ibrahim) are established and prescribed by the explicit hadith of Sahih Muslim and the general hadiths; the prayer of one who does so is valid and he is acting upon the Sunnah.
Translated from the original Urdu fatwa.