Raising ones finger during prayer has been reported from the Prophet, who “put his left hand on his left thigh and knee, and he put the edge of his right elbow on his right thigh, then he held two of his fingers together and made a circle, and raised his forefinger, and I saw him moving it and supplicating with it.” (An-Nasa’i, Sunan). It has been graded as authentic.

The Prophet has also been reported as raising his finger when he made supplication. On the authority of Sa`d ibn Abu Waqqas who said, “The Messenger of Allah (peace be upon him) passed by me when I was supplicating with my fingers and he said: ‘Make it one, make it one’ and pointed with his forefinger.” (Abu Dawud, Sunan, 1499). It has been graded as authentic.

He has also raised his fingers on delivering Friday sermon. `Umarah ibn Ruwaiba said he saw Bishr ibn Marwan on the pulpit raising his hands and said: Allah, disfigure these hands! I have seen Allah’s Messenger (peace be upon him) gesture no more than this with his hands, and he pointed with his forefinger.

 

Based on those reports, if someone suffers from disability which keeps him from pronouncing dhikr by mouth, he may mention Allah in his heart and refer to him in gestures through fingers.

Almighty Allah knows best.