In his book At-Tammam Al-Minnah, Al-Albani grades the Hadith of Malik bin Numayr Al-Khuza'i regarding the bending of the finger in the Tashahhud as being Da'if because his condition is unknown as per Adh-Dhahabi. However, Ibn Hajr in At-Taqrib graded him to be "acceptable" and other Hadith scholars have authenticated his narration including Ibn Khuzaymah, Ibn Hibban, Ali Za'i, Abu Dawud and Arna'uti (who said it is Sahih Li-Ghyrihi without this additional wording). If Malik bin Numayr is acceptable and no one contradicts him isn't his additional wording acceptable as well?

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, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

The additional phrase of the Hadeeth narrated by Maalik ibn Numayr is rejected because he is Layyin (i.e. lightly weak) in Hadeeth) as Al-Albaani asserted. As for the statement of Ibn Hajar that he is Maqbool (acceptable), it does not indicate that he is considered a reliable narrator or that his narration is accepted in an absolute manner, because Ibn Hajar stated in the introduction of his Taqreeb (i.e. his book Taqreeb At-Tahtheeb) that the transmission of a Maqbool transmitter is acceptable only if he is corroborated; otherwise, the narrator is considered Layyin Al-Hadeeth.

This additional wording was reported only by Maalik as Al-Albaani said in his book Dha‘eef Abu Daawood: “Its chain of narration is weak; Maalik ibn Numayr's condition is unknown and he is the only one who reported on the authority of his father as Ibn Al-Qattaan and Ath-Thahabi said…. He was the only one who narrated bending the index finger in the Tashahhud among all those who narrated lifting the finger in Tashahhud; therefore, it is a Munkar (denounced or objectionable) narration. He once reported the same narration without that additional wording as cited by Imaam Ahmad; this will be mentioned in detail later.”

Allaah Knows best.