The report which says one should say “ Wa ana
    ‘ala dhaalika min al-shaahideen (And I am one of those who testify to that)”
    after reciting Soorat al-Teen is da’eef (weak)
Question

                Assalaamu 'alaykum wa rahmatu 'Llahi wa
      barakatuhIs there a hadith that mentions after reciting the last ayah of surah
      At-Tin (#95) that one should say what means,"No doubt and I testify to it." Is
      that hadith authentic and where can I find it ?Jazzakallahu Khayrun
Praise be to Allah.Al-Tirmidhi, may Allaah have mercy on him, said in his Sunan:
    Ibn Abi Umar told us, Sufyaan told us from Ismaaeel ibn Umayyah who
    said: I heard a Bedouin Arab man say, I heard Abu Hurayrah say, narrating
    this, Whoever recites Wal-teeni wal-zaytoon (By the fig, and
    the olive) [al-Teen 95:1  interpretation of the meaning] then recites Alaysa
    Allaahu bi ahkam il haakimeen (Is not Allaah the Best of judges?) [al-Teen 95:8
     interpretation of the meaning], let him say, Balaa wa ana ala
    dhaalika min al-shaahideen (Yes indeed, and I am one of those who testify to
    that). Abu Eesa said: This hadeeth was narrated
    with this isnaad from this Bedouin from Abu Hurayrah, but his name is not given.
    Thus al-Tirmidhi (may Allaah have mercy on him) explained the fault in this isnaad, which
    is the fact that this Bedouin is not known (jahaalah). On this basis, the isnaad is
    daeef (weak), and the hadeeth is not saheeh. Abu Dawood (may Allaah have mercy on
    him) also narrated this report (753) from the same Bedouin. Al-Allaamah al-Albaani
    also classed the hadeeth as daeef in his book Daeef al-Jaami, no.
    5784. As the hadeeth is not proven, we cannot follow it. And Allaah knows best.