Ruling on pointing with the forefinger when supplicating
Question

Is it permissible to raise the forefinger to offer supplication or say ameen instead of raising the hands, especially if a person is unable to raise his hands, such as offering supplication whilst driving the car?
Praise be to Allah.It is proven that the Prophet (blessings and peace of Allah be upon him) pointed with his forefinger when offering supplication on the minbar on Friday. It was narrated from ‘Umaarah ibn Ru’aybah that he saw Bishr ibn Marwaan on the minbar raising his hands, and he said: How ugly are these two hands. I saw the Messenger of Allah (blessings and peace of Allah be upon him) doing no more than this with his hand, and he pointed with his index finger. Narrated by Muslim (874). 
Abu Dawood (1499) and an-Nasaa’i (1273) narrated that Sa‘d ibn Abi Waqqaas (may Allah be pleased with him) said: The Prophet (blessings and peace of Allah be upon him) passed by me when I was offering supplication (gesturing) with my two fingers, and he said, “Make it one, make it one,” and pointed with his forefinger. Classed as saheeh by al-Albaani in Saheeh Abi Dawood. 
In other words: gesture with one finger, because the One on Whom you are calling is One. This was stated by al-Mubaarakfoori in Tuhfat al-Ahwadhi, and by as-Sindi in his Haashiyah ‘ala Ibn Maajah. 
This hadith may be understood as meaning that this was whilst he was praying, or it may be understood as meaning that this was other than in prayer. After researching the matter, we have not come across any narration of the hadith to indicate that he was praying, and perhaps this is what led some of the scholars to interpret it as pointing with the forefinger when offering supplication in the prayer and otherwise.
Al-Mannaawi said in Fayd al-Qadeer (1/238):
Some of them claimed that this was in the tashahhud, but there is no evidence for that. End quote. 
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said, commenting on the hadith of Sa‘d referred to above: 
They said: What he meant was: point with one finger, for the One on Whom you are calling is One. This is a clear statement that the pointing is referring to Allah, as he said to him, “Make it one, make it one,” i.e., make it one finger. So if the pointing referred to anything other than Allah, it would make no difference whether it was one finger or more, thus it is known that because the pointing referred to Allah, and He is One God, he instructed him not to point except with one finger, not two. Similarly, the Sunan indicate that one is the point with one finger when offering supplication in the prayer, on the minbar on Friday, and in other cases. End quote. 
Bayaan Talbees al-Jahamiyyah (2/443). 
Al-Bahooti al-Hanbali (may Allah have mercy on him) said: 
He should also point with the right forefinger when offering supplication in prayer and otherwise. End quote. 
Kashshaaf al-Qinaa‘ (1/356-357) 
To sum up, pointing with the forefinger when offering supplication is one of the Sunnahs that were affirmed by the scholars. Although in most cases the practice of the Prophet (blessings and peace of Allah be upon him) was to raise his hands (in supplication), there is nothing wrong with following either of the two Sunnahs, according to whatever is easier. Ibn ‘Abbaas (may Allah be pleased with him) said: When asking of Allah, raise your hands level with your shoulders or thereabouts, and asking for forgiveness: point with one finger, and when beseeching Allah, raise both hands. Narrated by Abu Dawood (1489); classed as saheeh by al-Albaani in Saheeh Abi Dawood. 
Ibn Rajab al-Hanbali (may Allah have mercy on him) said: 
With regard to the manner in which the Prophet (blessings and peace of Allah be upon him) raised his hands when offering supplication, several ways have been narrated from him, including the report that he used to point with his forefinger only. It was narrated that he used to do that on the minbar, and he did it when riding his mount… And he (blessings and peace of Allah be upon him) raised his hands with their backs towards the qiblah as he was facing in that direction, and their palms were towards his face.
End quote from Jaami‘ al-‘Uloom wa’l-Hikam (p. 126, 127) 
And Allah knows best.