All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.
The Sunnah in bowing, regardless of whether one is standing or sitting on a chair or on the ground, is to place his hands on his knees with the fingers spread. Abu Humayd As-Saa‘idi, may Allah be pleased with him, said, “I remember the prayer of the Messenger of Allah, sallallahu ‘alayhi wa sallam, better than any one of you. I saw him, when he made Takbeer, raise his hands to the level of his shoulders, and when he bowed, he would place his hands firmly on his knees...” [Al-Bukhaari]
The version of Abu Daawood reads, “...And when he bowed, he clutched his knees with his palms, with his fingers spread...”
The version of At-Tirmithi reads, “...And he placed his hands on his knees as if he was clutching them.”
Zaad Al-Mustaqni' (a Hanbali book) reads, “Then he makes Takbeer while raising his hands and bows, placing his hands on his knees with the fingers spread...”
We did not find any scholar making a distinction between the person performing the prayer while standing and the one praying while sitting on the ground or on a chair in this regard. In addition, this action is merely recommended; if a person bows and places his hands on his knees with the fingers joined or places his hands on his thighs instead of his knees, the prayer is valid; however, he will not earn the reward for following the Sunnah of the Prophet, sallallahu ‘alayhi wa sallam.
As for the manner in which the fingers should be placed on the ground in prostration, if the person prostrates by nodding without touching the ground with his forehead, he is not required to place his hands on the ground. Therefore, there is no need to ask how his fingers should be. Mataalib Uli An-Nuha (a Hanbali book) reads:
“A person who cannot prostrate with his forehead is not required to place the rest of the (seven) body parts which should touch the ground during prostration on the ground because scholars rendered the forehead the main body part that should touch the ground during prostration, and the rest of the (seven) body parts follow it in this regard. Ibn ‘Umar, may Allah be pleased with him, narrated that the Prophet, sallallahu ‘alayhi wa sallam, said, ‘The hands prostrate as the face prostrates. When one of you puts his face (on the ground) he should put his hands too (on the ground), and when he raises it, he should raise them too.’” [Ahmad, Abu Daawood, and An-Nasaa'i]"
In any case, the act of the Sunnah for the person making prostration and placing his hands on the ground is to join his fingers.
For more benefit, please refer to fatwa 85270.
Allah knows best.